The correct answer is C because the API Gateway integration timeout is set to 15 seconds , while the Lambda function timeout is 20 seconds . If the Lambda function takes longer than 15 seconds to return a response, API Gateway will stop waiting and return an HTTP 504 Gateway Timeout to the client, even if the Lambda function itself has not yet timed out. This exactly matches the symptom described in the question.

The fact that there are no errors in the Lambda logs is also an important clue. If Lambda were failing internally, timing out at its own limit, or throwing exceptions, those events would typically appear in Lambda logs. Instead, the likely problem is that API Gateway is timing out first, before Lambda reaches its own timeout threshold or produces an error.

Option A is incorrect because reserved concurrency affects the number of concurrent executions available to Lambda, not the integration response wait time. Option B is incorrect because increasing the Lambda timeout beyond 20 seconds would not help if API Gateway is still configured to stop waiting after 15 seconds. Option D is incorrect because per-client throttling limits control request rates, not backend timeout behavior.

The proper fix is to increase the API Gateway integration timeout so that API Gateway can wait long enough for the Lambda function to complete. This ensures that requests that legitimately need more than 15 seconds can still return successful responses instead of 504 errors.