Your serverless architecture using AWS API Gateway, AWS Lambda, and AWS DynamoDB experienced a large increase in traffic to a sustained 400 requests per second, and dramatically increased in failure rates. Your requests, during normal operation, last 500 milliseconds on average. Your DynamoDB table did not exceed 50% of provisioned throughput, and Table primary keys are designed correctly. What is the most likely issue?

you can expect to support 200 requests per second at 100 concurrency. This is less than the 400 requests per second your system now requires. Make a limit increase request via the AWS Support Console. http://docs.aws.amazon.com/general/latest/gr/aws_service_limits.html#limits_lambda

A.
Your API Gateway deployment is throttling your requests.

B.
Your AWS API Gateway Deployment is bottlenecking on request (de)serialization.

C.
You did not request a limit increase on concurrent Lambda function executions.

D.
You used Consistent Read requests on DynamoDB and are experiencing semaphore lock.

Explanation:
Explanation/Reference:
AWS API Gateway by default throttles at 500 requests per second steady-state, and 1000 requests per second at spike.
Lambda, by default, throttles at 100 concurrent requests for safety. At 500 milliseconds (half of a second) per request,
AWS Lambda: Concurrent requests safety throttle per account -> 100