This SOA-C02 practice question tests your understanding of security and compliance. 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.
A SysOps administrator deploys the CloudFormation template shown in the exhibit. The stack creation fails with a security group error. 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 template uses 'SecurityGroups' instead of 'SecurityGroupIds' for a VPC instance.
The correct answer is C. In a VPC, EC2 instances require security groups to be specified using the 'SecurityGroupIds' property (a list of security group IDs) instead of 'SecurityGroups', which is used only in EC2-Classic. The 'SecurityGroups' property expects security group names, but when referencing a security group created in the same template via the Ref intrinsic function, it returns the security group ID. Using 'SecurityGroups' with a Ref in a VPC causes a mismatch and results in a security group error. Options A, B, and D are incorrect: A is not related to security groups, B mentions an invalid CIDR but 10.0.0.0/8 is a valid private range, and D describes a security risk but not a stack creation failure.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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 AMI ID is incorrect.
Why it's wrong here
The error is about security group.
✗
The security group ingress rule uses an invalid CIDR.
Why it's wrong here
10.0.0.0/8 is valid.
✓
The template uses 'SecurityGroups' instead of 'SecurityGroupIds' for a VPC instance.
Why this is correct
In VPC, you must use SecurityGroupIds.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The security group ingress rule allows SSH from all IPs.
Why it's wrong here
It allows from 10.0.0.0/8, not all.
Common exam traps
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.
TExam Day Tips
→Write the block size before choosing the subnet.
→Check whether the question asks for hosts, subnets or a specific address range.
→Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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
What to study next
Got this wrong? Here's your next step.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related SOA-C02 subnetting questions on CIDR, address ranges, and subnet selection.
Security and Compliance — This question tests Security and Compliance — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The template uses 'SecurityGroups' instead of 'SecurityGroupIds' for a VPC instance. — The correct answer is C. In a VPC, EC2 instances require security groups to be specified using the 'SecurityGroupIds' property (a list of security group IDs) instead of 'SecurityGroups', which is used only in EC2-Classic. The 'SecurityGroups' property expects security group names, but when referencing a security group created in the same template via the Ref intrinsic function, it returns the security group ID. Using 'SecurityGroups' with a Ref in a VPC causes a mismatch and results in a security group error. Options A, B, and D are incorrect: A is not related to security groups, B mentions an invalid CIDR but 10.0.0.0/8 is a valid private range, and D describes a security risk but not a stack creation failure.
What should I do if I get this SOA-C02 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related SOA-C02 subnetting questions on CIDR, address ranges, and subnet selection.
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?
CIDR notation defines the prefix length.
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