DOP-C02 Configuration Management and IaC Practice Question
This DOP-C02 practice question tests your understanding of configuration management and iac. 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
Refer to the exhibit.
Error log from CloudFormation stack creation:
"Property validation failure: The value for parameter "SecurityGroupIds" is not a list."
A CloudFormation template includes the following resource:
MySecurityGroup:
Type: AWS::EC2::SecurityGroup
Properties:
GroupDescription: My security group
SecurityGroupIngress:
- IpProtocol: tcp
FromPort: 443
ToPort: 443
CidrIp: 0.0.0.0/0
The stack creation fails with the error shown. What is the cause?
Exhibit
Refer to the exhibit.
Error log from CloudFormation stack creation:
"Property validation failure: The value for parameter "SecurityGroupIds" is not a list."
A
The SecurityGroupIds property must be a list, but !Ref returns a single value.
!Ref returns a string, not a list.
B
The SecurityGroupIds property must be a list of security group names, not IDs.
Why wrong: It expects IDs.
C
The security group ingress rule is invalid because it allows all traffic.
Why wrong: The ingress rule is valid; it allows HTTPS from anywhere.
D
The ImageId is missing, so the security group validation fails first.
Why wrong: The error specifically mentions SecurityGroupIds.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The SecurityGroupIds property must be a list, but !Ref returns a single value.
The error occurs because the `SecurityGroupIds` property expects a list of security group IDs, but the `!Ref` intrinsic function returns a single security group ID (a string), not a list. In CloudFormation, `!Ref` for a security group returns its ID as a scalar value, so wrapping it in a list (e.g., `[!Ref MySecurityGroup]`) is required to satisfy the `List<String>` type constraint.
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 SecurityGroupIds property must be a list, but !Ref returns a single value.
Why this is correct
!Ref returns a string, not a list.
Related concept
Read the scenario before looking for a memorised answer.
✗
The SecurityGroupIds property must be a list of security group names, not IDs.
The security group ingress rule is invalid because it allows all traffic.
Why it's wrong here
The ingress rule is valid; it allows HTTPS from anywhere.
✗
The ImageId is missing, so the security group validation fails first.
Why it's wrong here
The error specifically mentions SecurityGroupIds.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume `!Ref` automatically returns a list when the property expects one, but CloudFormation does not coerce scalar values into lists; you must explicitly provide a list literal.
Detailed technical explanation
How to think about this question
Under the hood, CloudFormation's `AWS::EC2::Instance` resource uses the `SecurityGroupIds` property to attach security groups by their EC2 resource IDs (e.g., sg-12345). The `!Ref` function for a security group returns the physical ID as a string, not a list. If you use `!GetAtt MySecurityGroup.GroupId`, it also returns a string. To pass a list, you must explicitly wrap it in square brackets: `SecurityGroupIds: [!Ref MySecurityGroup]`. This is a common pitfall because many other properties (e.g., `ImageId`) accept a single value, but `SecurityGroupIds` is typed as `List<AWS::EC2::SecurityGroup::Id>`.
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
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.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Configuration Management and IaC — This question tests Configuration Management and IaC — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The SecurityGroupIds property must be a list, but !Ref returns a single value. — The error occurs because the `SecurityGroupIds` property expects a list of security group IDs, but the `!Ref` intrinsic function returns a single security group ID (a string), not a list. In CloudFormation, `!Ref` for a security group returns its ID as a scalar value, so wrapping it in a list (e.g., `[!Ref MySecurityGroup]`) is required to satisfy the `List<String>` type constraint.
What should I do if I get this DOP-C02 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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