What functions as an intermediary server for requests from clients to back-end services?

The correct answer is the API Gateway. An API Gateway serves as an intermediary that facilitates communication between clients and backend services. It handles requests from clients, directing them to the appropriate microservices or back-end applications, which allows for central management of APIs. This is especially beneficial in microservices architectures where multiple back-end services are involved, as it streamlines API requests and offers additional capabilities such as rate limiting, authentication, and response transformation.

Load balancers, while also acting as intermediaries, primarily distribute incoming network traffic across multiple servers to ensure no single server becomes overloaded. A proxy server is similar in that it can forward requests but may not have the specific functionalities of managing APIs such as routing requests based on specific rules or aggregating responses from multiple services. An application server, on the other hand, is designed to host applications and run business logic, but it does not inherently manage API requests in the efficient manner that an API Gateway does. Therefore, the API Gateway is the most accurate choice for the role described in the question.