The answer is the Lambda function’s VPC configuration lacks an S3 VPC endpoint, cutting off network access to S3. When you attach a Lambda function to a VPC, it loses default internet connectivity, so any S3 API calls—like PutObject—fail with “Access Denied” because the function cannot reach the public S3 endpoint. To resolve this, you must add a gateway-type S3 VPC endpoint to the same VPC and route table, allowing private traffic to S3 without needing a NAT gateway. On the AWS Certified Solutions Architect Professional SAP-C02 exam, this scenario tests your understanding of how VPC isolation impacts Lambda’s ability to access AWS services—a common trap is assuming IAM permissions alone are sufficient. Remember the key insight: VPC Lambda loses internet by default, so always check for VPC endpoints or NAT when S3 access fails. Memory tip: “VPC Lambda + no endpoint = S3 dead end.”
SAP-C02 Continuous Improvement for Existing Solutions Practice Question
This SAP-C02 practice question tests your understanding of continuous improvement for existing solutions. 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 company runs a batch processing job on a schedule using AWS Lambda. The job processes files from an S3 bucket and writes results to another S3 bucket. Recently, the job has been failing with the error 'Access Denied' when trying to write to the destination bucket. The Lambda function's execution role has the following IAM policy attached:
The Lambda function also has a VPC configuration to access an RDS instance. The S3 buckets are in the same region. The Solutions Architect verified that the destination bucket policy does not deny access. What is the MOST likely cause of the 'Access Denied' error?
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 Lambda function is in a VPC without an S3 VPC endpoint, so it cannot reach S3.
Option D is correct. When a Lambda function is configured to access a VPC, it loses internet access unless a NATgateway or VPC endpoints are provided. To access S3, the function needs an S3 VPC endpoint (gateway type) in the same VPC and subnet. Without it, the PutObject call fails with 'Access Denied' because the function cannot reach the S3 API endpoint. Option A is wrong because the IAM policy allows s3:PutObject. Option B is wrong because the bucket policy does not deny access. Option C is wrong because the source bucket permissions are for read, not write; the error is on write.
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 Lambda function is in a VPC without an S3 VPC endpoint, so it cannot reach S3.
Why this is correct
Lambda in a VPC requires a VPC endpoint for S3 to access S3 APIs; without it, calls fail.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The Lambda function does not have permissions to read from the source bucket.
Why it's wrong here
The error is on write, not read; the function can read successfully.
✗
The IAM policy does not allow s3:PutObject on the destination bucket.
Why it's wrong here
The policy explicitly allows s3:PutObject on the destination bucket.
✗
The destination bucket policy denies the Lambda function's access.
Why it's wrong here
The scenario states the bucket policy does not deny access.
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.
Trap categories for this question
Scenario analysis trap
The scenario states the bucket policy does not deny access.
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 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 block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related SAP-C02 subnetting questions on CIDR, address ranges, and subnet selection.
Continuous Improvement for Existing Solutions — This question tests Continuous Improvement for Existing Solutions — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The Lambda function is in a VPC without an S3 VPC endpoint, so it cannot reach S3. — Option D is correct. When a Lambda function is configured to access a VPC, it loses internet access unless a NAT gateway or VPC endpoints are provided. To access S3, the function needs an S3 VPC endpoint (gateway type) in the same VPC and subnet. Without it, the PutObject call fails with 'Access Denied' because the function cannot reach the S3 API endpoint. Option A is wrong because the IAM policy allows s3:PutObject. Option B is wrong because the bucket policy does not deny access. Option C is wrong because the source bucket permissions are for read, not write; the error is on write.
What should I do if I get this SAP-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 SAP-C02 subnetting questions on CIDR, address ranges, and subnet selection.
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?
CIDR notation defines the prefix length.
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