- A
The network ACL for the private subnet does not allow inbound traffic from the ALB's security group.
Why wrong: Network ACLs cannot reference security groups; they use CIDR blocks.
- B
The security group for the ALB does not allow outbound traffic to the targets.
Why wrong: Security groups are stateful; inbound rule allowing traffic from ALB automatically allows outbound response.
- C
The ALB health check is configured to use HTTPS but the target only listens on HTTP.
This would cause the health check to fail with a timeout or non-200 response, resulting in 503.
- D
The route table for the private subnet does not have a route to the ALB's subnet.
Why wrong: This would cause complete lack of connectivity, not just health check failures.
Question 1,445 of 1,705
ANS-C01 Application Load Balancer Health Checks Practice Question
This ANS-C01 practice question tests your understanding of network implementation. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. A key principle to apply: application Load Balancer Health Checks. 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.
A company has a VPC with CIDR 10.0.0.0/16. They have two subnets: a public subnet (10.0.1.0/24) and a private subnet (10.0.2.0/24). An Application Load Balancer (ALB) is deployed in the public subnet, and EC2 instances are in the private subnet. The ALB has a target group pointing to the EC2 instances. The security group for the EC2 instances allows traffic from the ALB's security group on port 80. The network ACL for the private subnet allows inbound traffic on port 80 from the public subnet CIDR (10.0.1.0/24) and allows outbound ephemeral ports. However, the ALB health checks are failing with 503 errors. The network engineer checks the ALB logs and sees that TCP connections are established but HTTP requests are timing out. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"most likely"Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The ALB health check is configured to use HTTPS but the target only listens on HTTP.
The ALB health check is configured to use HTTPS, but the target only listens on HTTP. This mismatch causes the health check to fail because the target cannot respond to HTTPS requests, leading to a 503 error. The network ACL and security group configurations appear correct: inbound traffic from the public subnet CIDR is allowed, outbound ephemeral ports are allowed, and security groups are stateful. Option D is incorrect because network ACLs cannot reference security groups. Option A is unlikely because security groups are stateful. Option C would cause complete connectivity failure, not just health check issues.
Key principle: Application Load Balancer Health Checks
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The network ACL for the private subnet does not allow inbound traffic from the ALB's security group.
Why it's wrong here
Network ACLs cannot reference security groups; they use CIDR blocks.
- ✗
The security group for the ALB does not allow outbound traffic to the targets.
Why it's wrong here
Security groups are stateful; inbound rule allowing traffic from ALB automatically allows outbound response.
- ✓
The ALB health check is configured to use HTTPS but the target only listens on HTTP.
Why this is correct
This would cause the health check to fail with a timeout or non-200 response, resulting in 503.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Application Load Balancer Health Checks
- ✗
The route table for the private subnet does not have a route to the ALB's subnet.
Why it's wrong here
This would cause complete lack of connectivity, not just health check failures.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The common pitfall is misconfiguring health check protocol to HTTPS when targets only support HTTP, causing 503 errors even though TCP connections succeed.
Detailed technical explanation
How to think about this question
Treat this as a scenario question. Identify the problem, the constraint, and the best action. Then compare each option against those facts.
KKey Concepts to Remember
- Application Load Balancer Health Checks
- Network ACL vs Security Group
- Stateful vs Stateless Firewalls
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
Application Load Balancer Health Checks
Real-world example
How this comes up in practice
A healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
Visual reference
Quick reference
Cloud Service Model Comparison
| Model | You Manage | Provider Manages | Examples |
|---|---|---|---|
| IaaS | OS, runtime, apps, data | Hardware, hypervisor, networking | EC2, Azure VMs, GCP Compute Engine |
| PaaS | Apps and data | OS, runtime, middleware, hardware | Elastic Beanstalk, Azure App Service |
| SaaS | Data and settings only | Everything else | Microsoft 365, Salesforce, Workday |
| FaaS / Serverless | Function code only | Infra, scaling, runtime | Lambda, Azure Functions, Cloud Run |
| CaaS | Containers and apps | Kubernetes, OS, hardware | EKS, AKS, GKE |
What to study next
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FAQ
Questions learners often ask
What does this ANS-C01 question test?
Network Implementation — This question tests Network Implementation — Application Load Balancer Health Checks.
What is the correct answer to this question?
The correct answer is: The ALB health check is configured to use HTTPS but the target only listens on HTTP. — The ALB health check is configured to use HTTPS, but the target only listens on HTTP. This mismatch causes the health check to fail because the target cannot respond to HTTPS requests, leading to a 503 error. The network ACL and security group configurations appear correct: inbound traffic from the public subnet CIDR is allowed, outbound ephemeral ports are allowed, and security groups are stateful. Option D is incorrect because network ACLs cannot reference security groups. Option A is unlikely because security groups are stateful. Option C would cause complete connectivity failure, not just health check issues.
What should I do if I get this ANS-C01 question wrong?
Review application Load Balancer Health Checks, then practise related ANS-C01 questions on the same topic to reinforce the concept.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Application Load Balancer Health Checks
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Last reviewed: Jun 20, 2026
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