Which protocol is primarily used for communication between APIs?

The correct response is HTTP/HTTPS because these protocols are the foundation of web communication and are extensively used for the request-response model of how APIs interact over the internet. HTTP (Hypertext Transfer Protocol) is designed for transmitting hypermedia documents, and when secured with SSL/TLS, it becomes HTTPS (HTTP Secure). This secure version is essential for protecting data transmitted between clients and servers, making it the most common choice for API communication.

APIs rely on the principles of the web, where resources are identified using URIs, and operations are performed using standard HTTP methods like GET, POST, PUT, and DELETE. These methods facilitate CRUD operations (Create, Read, Update, Delete) on resources, thus streamlining the interaction between clients and servers.

While protocols like FTP (File Transfer Protocol) are designed for transferring files, SMTP (Simple Mail Transfer Protocol) is used primarily for sending emails, and WebSocket provides a two-way interactive communication channel, they do not serve the fundamental role that HTTP/HTTPS plays in API communication. Therefore, HTTP/HTTPS is the most relevant protocol when it comes to the operation and design of APIs.