A company has a legacy mainframe system that can retrieve data only from systems that...

The correct answer is A because the legacy mainframe requires a synchronous RESTful API and the new service takes approximately 3 minutes to complete each request. Among the choices, Amazon API Gateway REST API is the appropriate option for exposing a synchronous REST endpoint to the mainframe. API Gateway REST APIs support longer integration timeouts than HTTP APIs, which makes them more suitable for workloads that take an extended time to process. The REST API can invoke an AWS Lambda function to execute the stock price calculation and return the result synchronously.

Option B is incorrect because API Gateway HTTP APIs have a shorter integration timeout and are not the best fit for a request that takes around 3 minutes. Option C is incorrect because WebSocket APIs are not RESTful and do not meet the requirement for synchronous REST-based integration with the mainframe. Option D is incorrect because Lambda function URLs do not provide the same API management features and are not the best choice for a long-running synchronous enterprise integration requirement of this kind.

AWS design guidance favors API Gateway when exposing managed APIs to external consumers. In this case, the REST API option best matches the requirement for synchronous RESTful access while supporting the longer processing duration. It also provides a managed front door for authentication, throttling, and operational control if the company needs those features later.

One note of honesty: some exam versions treat the REST API timeout detail as the deciding factor here. That is the key reason REST API is preferred over HTTP API for this question.