The correct answer is that `!Ref MySecurityGroup` returns a single string, but `SecurityGroupIds` requires a list of strings. When you use `!Ref` on an `AWS::EC2::SecurityGroup` resource, CloudFormation returns the security group’s physical ID as a plain string value, not a list. The `SecurityGroupIds` property explicitly expects an array, so passing a single string causes the stack creation to fail with a validation error. On the AWS Certified DevOps Engineer Professional DOP-C02 exam, this tests your understanding of how CloudFormation intrinsic functions handle return types, especially the common trap of forgetting that `!Ref` returns a scalar for most resources. A frequent memory tip is to always wrap `!Ref` in square brackets when the property expects a list—for example, `[!Ref MySecurityGroup]`—or use `!Sub` with a list literal. Remember: if the property name ends in “Ids” or “Groups,” it almost always expects a list, not a single value.
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
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.
Option C is correct. The SecurityGroupIds property expects a list of security group IDs, but the !Ref MySecurityGroup returns a single string (the security group ID). To pass a list, you must use !Sub or a list literal like [!Ref MySecurityGroup]. Option A is incorrect because the security group definition is correct. Option B is incorrect because security groups can be referenced by ID. Option D is incorrect because the error is about format, not missing property.
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 SecurityGroupIds property must be a list, but !Ref returns a single value.
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: 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.
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 DOP-C02 subnetting questions on CIDR, address ranges, and subnet selection.
Configuration Management and IaC — This question tests Configuration Management and IaC — CIDR notation defines the prefix length..
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. — Option C is correct. The SecurityGroupIds property expects a list of security group IDs, but the !Ref MySecurityGroup returns a single string (the security group ID). To pass a list, you must use !Sub or a list literal like [!Ref MySecurityGroup]. Option A is incorrect because the security group definition is correct. Option B is incorrect because security groups can be referenced by ID. Option D is incorrect because the error is about format, not missing property.
What should I do if I get this DOP-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 DOP-C02 subnetting questions on CIDR, address ranges, and subnet selection.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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