Which protocol does the REST API use?

The REST API primarily utilizes the HTTP protocol for communication. This is due to the nature of REST (Representational State Transfer), which relies on standard web technologies and principles to facilitate stateless interactions between clients and servers. HTTP supports several methods (such as GET, POST, PUT, DELETE) that align well with RESTful operations, making it the ideal protocol for implementing REST APIs.

Each of the other listed protocols serves different purposes. For instance, FTP (File Transfer Protocol) is used for transferring files between hosts on a network but does not provide the statelessness or structured interactions typical of REST APIs. SMTP (Simple Mail Transfer Protocol) is designed for sending emails and not for resource manipulation as REST APIs require. POP3 (Post Office Protocol version 3) is another email protocol used primarily for retrieving emails from a server, which also does not align with the functionalities of a RESTful API. Thus, HTTP stands out as the correct choice for REST API implementations.