The answer is an explicit deny in the IAM policy for s3:PutObject on the 'sensitive/' prefix. This is because, in AWS IAM, an explicit deny always overrides any allow statement, regardless of the order in which they appear. Even though the first statement grants s3:PutObject on the entire 'data-lake-bucket', the second statement specifically denies that action on the 'sensitive/' prefix, which is why the Glue job fails when writing there. On the MLS-C01 exam, this tests your understanding of IAM policy evaluation logic, a common trap where candidates assume a broad allow will cover a specific deny. A key memory tip is "Explicit Deny is the ultimate veto"—once an explicit deny is hit, no other policy can override it.
MLS-C01 Data Engineering Practice Question
This MLS-C01 practice question tests your understanding of data engineering. 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.
A data engineer is troubleshooting an AWS Glue job that reads from and writes to the S3 bucket 'data-lake-bucket'. The job fails when trying to write to the 'sensitive/' prefix. The IAM policy attached to the Glue job's IAM role is shown in the exhibit. What is the MOST likely reason for the failure?
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 IAM role has an explicit deny for s3:PutObject on the 'sensitive/' prefix
Option B is correct. Even though the first statement allows s3:PutObject on the entire bucket, the second statement explicitly denies s3:PutObject on the 'sensitive/' prefix. Explicit deny overrides any allow. Option A is wrong because the policy allows GetObject. Option C is wrong because the policy covers the bucket. Option D is wrong because there is a deny statement.
Key principle: ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
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 IAM role does not have permission to read objects from the bucket
Why it's wrong here
The policy allows s3:GetObject on all objects in the bucket.
✓
The IAM role has an explicit deny for s3:PutObject on the 'sensitive/' prefix
Why this is correct
The Deny statement blocks write access to the sensitive prefix.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Standard ACLs match source addresses.
✗
The IAM policy does not specify the bucket resource correctly
Why it's wrong here
The resource ARN is correctly specified for objects in the bucket.
✗
The IAM policy lacks a required condition for encryption
Why it's wrong here
There is no condition required; the deny is explicit.
Common exam traps
Common exam trap: ACLs stop at the first match
ACLs are processed top to bottom. The first matching entry wins, and an implicit deny usually exists at the end.
Detailed technical explanation
How to think about this question
ACL questions test precision: source, destination, protocol, port and direction. A generally correct ACL can still fail if it is applied on the wrong interface or in the wrong direction.
KKey Concepts to Remember
Standard ACLs match source addresses.
Extended ACLs can match source, destination, protocol and ports.
The first matching ACL entry is used.
There is usually an implicit deny at the end.
TExam Day Tips
→Check inbound versus outbound direction.
→Read the ACL from top to bottom.
→Look for a broader permit or deny above the intended line.
Key takeaway
ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
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.
What to study next
Got this wrong? Here's your next step.
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related MLS-C01 ACL questions on filtering logic and placement.
Data Engineering — This question tests Data Engineering — Standard ACLs match source addresses..
What is the correct answer to this question?
The correct answer is: The IAM role has an explicit deny for s3:PutObject on the 'sensitive/' prefix — Option B is correct. Even though the first statement allows s3:PutObject on the entire bucket, the second statement explicitly denies s3:PutObject on the 'sensitive/' prefix. Explicit deny overrides any allow. Option A is wrong because the policy allows GetObject. Option C is wrong because the policy covers the bucket. Option D is wrong because there is a deny statement.
What should I do if I get this MLS-C01 question wrong?
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related MLS-C01 ACL questions on filtering logic and placement.
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?
Standard ACLs match source addresses.
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