Which communication method provides a full-duplex channel over a single connection?

The correct choice is Web Sockets, which is designed specifically to provide a full-duplex communication channel over a single connection. Full-duplex means that data can be sent and received simultaneously, allowing for real-time communication. Web Sockets establish a persistent connection between a client and server, enabling both parties to send messages to each other continuously without the need to re-establish a connection for each message. This makes Web Sockets particularly useful for applications that require instant data exchange, such as live chats, online gaming, and real-time notifications.

In contrast, the other options serve different communication purposes and do not support full-duplex communications in the same way. Protocols like HTTP and FTP work on a request-and-response basis, where the client must wait for a response before sending another request, which does not allow for simultaneous bidirectional data flow. MQTT is primarily designed for lightweight messaging and operates on a publish/subscribe model but typically does not provide the same level of real-time interaction as Web Sockets. Thus, Web Sockets stand out for their ability to facilitate efficient and instant communication between users and servers.