Which query language is specifically designed for APIs to facilitate efficient data transfer?

The correct choice is GraphQL, which is specifically designed to facilitate efficient data transfer between APIs and clients. Unlike traditional REST APIs, where the server defines the structure and amount of data returned, GraphQL empowers clients to request exactly the data they need. This is accomplished through a single endpoint that can handle various types of queries, allowing for more flexible and efficient communication.

GraphQL's query language allows clients to specify the shape of the response, minimizing the amount of data transferred over the network and reducing the number of requests needed to gather relevant information. This capability significantly improves performance, especially in applications with complex data schemas or multiple interrelated resources.

In contrast, REST relies on predefined endpoints and resource representations, which can lead to over-fetching or under-fetching of data. SOAP is a protocol that relies on XML for messaging between services and is typically heavier than REST or GraphQL. SQL, while a powerful query language for interacting with databases, is not designed for API interactions but rather for querying and managing data within database systems.