This DOP-C02 practice question tests your understanding of sdlc automation. 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 team uses AWS CloudFormation to manage infrastructure. They want to deploy a stack that creates an S3 bucket and a DynamoDB table. The S3 bucket name must be unique across all AWS accounts. Which CloudFormation intrinsic function should be used to generate a unique bucket name?
!Ref 'AWS::StackName'
Why wrong: StackName may not be unique across accounts.
!GetAtt S3Bucket.Arn
Why wrong: Arn is generated after creation and is not available during template evaluation for naming.
!Sub 'mybucket-${AWS::AccountId}'
AccountId is globally unique, ensuring bucket name uniqueness.
!Select [0, !Split ['-', !Ref 'AWS::Region']]
Why wrong: This extracts part of the region name, which is not unique.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
!Sub 'mybucket-${AWS::AccountId}'
Option C is correct because the `!Sub 'mybucket-${AWS::AccountId}'` intrinsic function substitutes the AWS::AccountId pseudo parameter, which is guaranteed to be unique per AWS account. Since S3 bucket names must be globally unique across all AWS accounts, appending the account ID ensures the generated name does not conflict with buckets in other accounts. This approach is a common pattern for creating unique resource names in CloudFormation.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
!Ref 'AWS::StackName'
Why it's wrong here
StackName may not be unique across accounts.
!GetAtt S3Bucket.Arn
Why it's wrong here
Arn is generated after creation and is not available during template evaluation for naming.
!Sub 'mybucket-${AWS::AccountId}'
Why this is correct
AccountId is globally unique, ensuring bucket name uniqueness.
Related concept
Read the scenario before looking for a memorised answer.
!Select [0, !Split ['-', !Ref 'AWS::Region']]
Why it's wrong here
This extracts part of the region name, which is not unique.
Common exam traps
The trap here is that candidates may think `!Ref 'AWS::StackName'` or `!Ref 'AWS::Region'` provide sufficient uniqueness, but they overlook the requirement for global uniqueness across all AWS accounts, which only `AWS::AccountId` guarantees.
Detailed technical explanation
Under the hood, CloudFormation pseudo parameters like `AWS::AccountId` are resolved at stack creation time and are immutable for the stack's lifetime. The `!Sub` function performs string substitution before resource creation, allowing the bucket name to be constructed deterministically. In real-world scenarios, you might combine `AWS::AccountId` with `AWS::Region` or a random suffix to further reduce collision risk, but for global uniqueness, the account ID alone is sufficient because no two AWS accounts share the same ID.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.
What to study next
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FAQ
SDLC Automation — This question tests SDLC Automation — Read the scenario before looking for a memorised answer..
The correct answer is: !Sub 'mybucket-${AWS::AccountId}' — Option C is correct because the `!Sub 'mybucket-${AWS::AccountId}'` intrinsic function substitutes the AWS::AccountId pseudo parameter, which is guaranteed to be unique per AWS account. Since S3 bucket names must be globally unique across all AWS accounts, appending the account ID ensures the generated name does not conflict with buckets in other accounts. This approach is a common pattern for creating unique resource names in CloudFormation.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 2026
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.
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