SAP-C02 Design for New Solutions Practice Question
This SAP-C02 practice question tests your understanding of design for new solutions. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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 solutions architect attempts to create this stack but receives an error: "Value of property SecurityGroups must be a list of strings". What is the likely cause?
The SecurityGroups property should be a list, but the YAML specifies a single reference incorrectly.
The error occurs when the SecurityGroups property is provided as a single string instead of a list. In CloudFormation, even one security group must be specified as a list, e.g., [!Ref MySecurityGroup].
B
The security group ingress rule allows SSH from anywhere.
Why wrong: The ingress rule allowing SSH from anywhere is a separate security group configuration, not the cause of the 'list of strings' error.
C
There is a circular dependency between the EC2 instance and the security group.
Why wrong: A circular dependency would cause a different error, such as a stack update failure, not the SecurityGroups list type error.
D
The AMI ID is invalid.
Why wrong: An invalid AMI ID would result in an error about the AMI, not about the SecurityGroups property type.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The SecurityGroups property should be a list, but the YAML specifies a single reference incorrectly.
The error 'Value of property SecurityGroups must be a list of strings' occurs because the YAML template specifies the SecurityGroups property as a single string (e.g., !Ref MySecurityGroup) instead of a list of strings (e.g., [!Ref MySecurityGroup]). In AWS CloudFormation, the SecurityGroups property for an EC2 instance expects a list of security group IDs or names, even if only one security group is provided. The YAML syntax must wrap the reference in square brackets to form a list, or the template will fail validation.
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 SecurityGroups property should be a list, but the YAML specifies a single reference incorrectly.
Why this is correct
The error occurs when the SecurityGroups property is provided as a single string instead of a list. In CloudFormation, even one security group must be specified as a list, e.g., [!Ref MySecurityGroup].
Related concept
Read the scenario before looking for a memorised answer.
✗
The security group ingress rule allows SSH from anywhere.
Why it's wrong here
The ingress rule allowing SSH from anywhere is a separate security group configuration, not the cause of the 'list of strings' error.
✗
There is a circular dependency between the EC2 instance and the security group.
Why it's wrong here
A circular dependency would cause a different error, such as a stack update failure, not the SecurityGroups list type error.
✗
The AMI ID is invalid.
Why it's wrong here
An invalid AMI ID would result in an error about the AMI, not about the SecurityGroups property type.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates may confuse the SecurityGroups property with SecurityGroupIds, or assume that a single reference can be passed as a scalar, but CloudFormation strictly enforces the list type for SecurityGroups even when only one security group is used.
Detailed technical explanation
How to think about this question
In AWS CloudFormation, the AWS::EC2::Instance resource's SecurityGroups property accepts a list of strings, where each string is either a security group ID (sg-xxx) or a security group name (for EC2-Classic). When using YAML, a single value must be wrapped in brackets (e.g., [ !Ref MySecurityGroup ]) to be interpreted as a list; otherwise, YAML treats it as a scalar string. This is a common pitfall because the AWS::EC2::Instance resource also supports a SecurityGroupIds property (which requires a list of IDs), but SecurityGroups is the older property that still expects a list. The error is caught during template validation before any resources are created.
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.
Design for New Solutions — This question tests Design for New Solutions — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The SecurityGroups property should be a list, but the YAML specifies a single reference incorrectly. — The error 'Value of property SecurityGroups must be a list of strings' occurs because the YAML template specifies the SecurityGroups property as a single string (e.g., !Ref MySecurityGroup) instead of a list of strings (e.g., [!Ref MySecurityGroup]). In AWS CloudFormation, the SecurityGroups property for an EC2 instance expects a list of security group IDs or names, even if only one security group is provided. The YAML syntax must wrap the reference in square brackets to form a list, or the template will fail validation.
What should I do if I get this SAP-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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