The answer is that the security group for the EC2 instances does not allow inbound traffic from the ALB. This is the most likely cause because the ALB health check sends requests to the instance’s health check endpoint (typically on the configured port and path), and if the instance’s security group lacks a rule permitting traffic from the ALB’s source IP range or the ALB’s security group, those requests are silently dropped. From the OS perspective, the application and network stack are running fine, but the ALB never receives a response, so it marks the instance as unhealthy. On the AWS Certified DevOps Engineer Professional DOP-C02 exam, this scenario tests your understanding of how security group rules interact with load balancer health checks—a common trap is assuming the issue is with the application or the target group configuration when it is purely a network access control problem. Remember the memory tip: “Health checks are just traffic too—if the SG doesn’t say yes, the ALB says no.”
DOP-C02 Resilient Cloud Solutions Practice Question
This DOP-C02 practice question tests your understanding of resilient cloud solutions. 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.
An application on EC2 instances in an Auto Scaling group uses an ALB. The ALB health checks are failing for some instances, but the instances are healthy from the OS perspective. 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 the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The security group for the instances does not allow traffic from the ALB
Misconfigured security group rules can block health check traffic, causing the ALB to mark instances as unhealthy.
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.
✗
The ALB idle timeout is too low
Why it's wrong here
Idle timeout affects connections, not health checks.
✓
The security group for the instances does not allow traffic from the ALB
Why this is correct
If the security group blocks health check traffic, the ALB marks instances unhealthy.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The Auto Scaling group cooldown period is too short
Why it's wrong here
Cooldown does not affect health checks.
✗
The ALB cross-zone load balancing is disabled
Why it's wrong here
Cross-zone load balancing does not affect health checks.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.
Detailed technical explanation
How to think about this question
This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.
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.
Use explanations to understand the rule behind the answer.
TExam Day Tips
→Underline the problem statement mentally.
→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
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.
What to study next
Got this wrong? Here's your next step.
Identify which DOP-C02 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
Resilient Cloud Solutions — This question tests Resilient Cloud Solutions — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The security group for the instances does not allow traffic from the ALB — Misconfigured security group rules can block health check traffic, causing the ALB to mark instances as unhealthy.
What should I do if I get this DOP-C02 question wrong?
Identify which DOP-C02 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
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?
Read the scenario before looking for a memorised answer.
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This DOP-C02 practice question is part of Courseiva's free Amazon Web Services certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the DOP-C02 exam.
Question Discussion
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