This DVA-C02 practice question tests your understanding of development with aws services. 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.
Refer to the exhibit. An IAM policy is attached to a Lambda function's execution role. The Lambda function is triggered by an S3 event and needs to invoke another Lambda function and decrypt an S3 object encrypted with a customer-managed KMS key. However, the invocation fails with an access denied 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 policy is missing kms:GenerateDataKey and kms:Decrypt might not be sufficient for all decryption scenarios.
Option C is correct because the Lambda function's execution role policy includes `kms:Decrypt` but not `kms:GenerateDataKey`. When decrypting an S3 object encrypted with a customer-managed KMS key, the S3 service may need to call `kms:GenerateDataKey` to re-encrypt the object during certain operations (e.g., cross-region replication or copy), and the Lambda function itself might need to generate a data key for envelope encryption/decryption workflows. Without `kms:GenerateDataKey`, the decryption operation fails with an access denied error, even though `kms:Decrypt` is present.
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 policy does not allow s3:GetObject on the S3 bucket.
Why it's wrong here
The error is about invocation, not S3 access.
✗
The policy resource for Lambda is incorrect.
Why it's wrong here
The ARN is correct for the target function.
✓
The policy is missing kms:GenerateDataKey and kms:Decrypt might not be sufficient for all decryption scenarios.
Why this is correct
May need additional KMS actions like kms:DescribeKey.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The policy does not allow kms:Decrypt for the key.
Why it's wrong here
The policy does allow kms:Decrypt.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume `kms:Decrypt` alone is sufficient for all decryption scenarios, but AWS KMS often requires `kms:GenerateDataKey` for operations that involve re-encryption or envelope encryption, especially when the Lambda function interacts with S3 objects encrypted with customer-managed KMS keys.
Detailed technical explanation
How to think about this question
AWS KMS supports two types of operations: symmetric and asymmetric key operations. For envelope encryption, `GenerateDataKey` is used to create a data key that is encrypted under a KMS key, and `Decrypt` is used to decrypt that data key. In S3 event-driven Lambda workflows, the Lambda function may need to decrypt the S3 object's data key (which is stored alongside the object) using `kms:Decrypt`, but if the function also needs to re-encrypt the data for further processing (e.g., writing to another S3 bucket with a different KMS key), it requires `kms:GenerateDataKey`. The access denied error occurs because the IAM policy evaluation denies the `GenerateDataKey` call, which is implicitly required by the SDK or service integration.
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 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.
Quick reference
AWS S3 Storage Class Comparison
Storage Class
Min Duration
Retrieval
Use Case
S3 Standard
None
Immediate
Frequently accessed data
S3 Standard-IA
30 days
Immediate
Infrequent access, rapid retrieval
S3 One Zone-IA
30 days
Immediate
Non-critical infrequent data
S3 Intelligent-Tiering
None
Immediate–hours
Unknown or changing access patterns
S3 Glacier Instant
90 days
Milliseconds
Archive with instant retrieval
S3 Glacier Flexible
90 days
Minutes–hours
Archive, flexible retrieval
S3 Glacier Deep Archive
180 days
Hours
Long-term compliance archive
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.
Development with AWS Services — This question tests Development with AWS Services — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The policy is missing kms:GenerateDataKey and kms:Decrypt might not be sufficient for all decryption scenarios. — Option C is correct because the Lambda function's execution role policy includes `kms:Decrypt` but not `kms:GenerateDataKey`. When decrypting an S3 object encrypted with a customer-managed KMS key, the S3 service may need to call `kms:GenerateDataKey` to re-encrypt the object during certain operations (e.g., cross-region replication or copy), and the Lambda function itself might need to generate a data key for envelope encryption/decryption workflows. Without `kms:GenerateDataKey`, the decryption operation fails with an access denied error, even though `kms:Decrypt` is present.
What should I do if I get this DVA-C02 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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?
Read the scenario before looking for a memorised answer.
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Question Discussion
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