The answer is to add the unused subnet in us-east-1b to the Auto Scaling group. This is correct because an Auto Scaling group must span at least two Availability Zones to provide multi-AZ high availability; by adding the unused subnet, EC2 instances can launch in both AZs, and the Application Load Balancer can route traffic to healthy instances if one AZ fails. On the SAA-C03 exam, this scenario tests your understanding of fault isolation and the least operational overhead approach—the trap is overcomplicating the fix by suggesting new subnets, NAT gateways, or separate Auto Scaling groups when simply updating the group’s subnet list is sufficient. Remember the memory tip: “Add the AZ, not the complexity”—if a subnet already exists and is unused, the simplest HA fix is to include it in the Auto Scaling group’s configuration.
SAA-C03 Design Resilient Architectures Practice Question
This SAA-C03 practice question tests your understanding of design resilient architectures. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
Exhibit
ALB target group: 2 healthy targets in us-east-1a only
Auto Scaling group subnets: subnet-0a1b2c3d (us-east-1a)
Desired capacity: 2
Unused subnet available: subnet-9f8e7d6c (us-east-1b)
Health checks: passing
Recent incident note: "If us-east-1a is unavailable, both app instances are lost."
Based on the exhibit, the web team wants the application to continue serving traffic if one Availability Zone fails. Which change best meets the requirement with the least operational overhead?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "best"
Why it matters: Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
Clue: "least"
Why it matters: You want the option with minimum overhead, fewest steps, or lowest impact — not the most feature-rich or comprehensive answer.
ALB target group: 2 healthy targets in us-east-1a only
Auto Scaling group subnets: subnet-0a1b2c3d (us-east-1a)
Desired capacity: 2
Unused subnet available: subnet-9f8e7d6c (us-east-1b)
Health checks: passing
Recent incident note: "If us-east-1a is unavailable, both app instances are lost."
A
Increase desired capacity to 3 in the same Availability Zone so one extra instance is always available.
Why wrong: Adding more instances in the same Availability Zone increases capacity, but it does not remove the single-AZ dependency. If us-east-1a fails, all instances in that zone are still lost, so the application remains unavailable.
B
Add the unused subnet in us-east-1b to the Auto Scaling group so instances can launch in both AZs.
Placing the Auto Scaling group in at least two Availability Zones allows AWS to distribute and replace instances across zones. Because the Application Load Balancer can route only to healthy targets, adding the second subnet is the lowest-complexity change that gives the application resilience to a full AZ outage.
C
Replace the Application Load Balancer with a Network Load Balancer because it will automatically keep the app online.
Why wrong: Changing load balancer type does not solve the underlying issue. The application still needs healthy compute in more than one Availability Zone; otherwise, any full AZ outage removes all backend capacity.
D
Move the application to a larger EC2 instance type so a single server can handle the full workload.
Why wrong: A larger instance might handle more traffic, but it is still a single point of failure. This improves vertical capacity, not Availability Zone resilience.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Add the unused subnet in us-east-1b to the Auto Scaling group so instances can launch in both AZs.
Option B is correct because it adds the unused subnet in us-east-1b to the Auto Scaling group, enabling EC2 instances to launch across two Availability Zones. This provides fault isolation: if one AZ fails, the ALB can route traffic to healthy instances in the other AZ. The change requires only a configuration update to the Auto Scaling group, minimizing operational overhead while meeting the high-availability requirement.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Increase desired capacity to 3 in the same Availability Zone so one extra instance is always available.
Why it's wrong here
Adding more instances in the same Availability Zone increases capacity, but it does not remove the single-AZ dependency. If us-east-1a fails, all instances in that zone are still lost, so the application remains unavailable.
✓
Add the unused subnet in us-east-1b to the Auto Scaling group so instances can launch in both AZs.
Why this is correct
Placing the Auto Scaling group in at least two Availability Zones allows AWS to distribute and replace instances across zones. Because the Application Load Balancer can route only to healthy targets, adding the second subnet is the lowest-complexity change that gives the application resilience to a full AZ outage.
Clue confirmation
The clue words "best", "least" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Replace the Application Load Balancer with a Network Load Balancer because it will automatically keep the app online.
Why it's wrong here
Changing load balancer type does not solve the underlying issue. The application still needs healthy compute in more than one Availability Zone; otherwise, any full AZ outage removes all backend capacity.
✗
Move the application to a larger EC2 instance type so a single server can handle the full workload.
Why it's wrong here
A larger instance might handle more traffic, but it is still a single point of failure. This improves vertical capacity, not Availability Zone resilience.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume increasing instance count in a single AZ or using a different load balancer type alone provides high availability, but true resilience requires distributing instances across multiple Availability Zones.
Detailed technical explanation
How to think about this question
An Auto Scaling group can span multiple Availability Zones within a region, and the ALB distributes traffic across healthy targets in those AZs. When an AZ fails, the ALB automatically stops routing to instances in that AZ, and the Auto Scaling group launches replacement instances in the remaining AZs if needed. This pattern leverages the AWS global infrastructure's fault isolation boundaries, where each AZ is physically separate with independent power, cooling, and networking.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
An e-commerce site experiences heavy traffic on Black Friday and near-zero traffic during off-peak weeks. Rather than provisioning permanent large VMs, the team uses auto-scaling groups that add capacity automatically under load and reduce it overnight. Questions like this test whether you understand elasticity, availability zones, and cloud compute scaling patterns.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Design Resilient Architectures — This question tests Design Resilient Architectures — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Add the unused subnet in us-east-1b to the Auto Scaling group so instances can launch in both AZs. — Option B is correct because it adds the unused subnet in us-east-1b to the Auto Scaling group, enabling EC2 instances to launch across two Availability Zones. This provides fault isolation: if one AZ fails, the ALB can route traffic to healthy instances in the other AZ. The change requires only a configuration update to the Auto Scaling group, minimizing operational overhead while meeting the high-availability requirement.
What should I do if I get this SAA-C03 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "best", "least". Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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