Which one of these is not a common protocol?

In the context of networking protocols, TCP, or Transmission Control Protocol, is a foundational protocol that operates at the transport layer of the Internet Protocol Suite. It is responsible for establishing connections and ensuring reliable data transmission between devices. Because TCP is a fundamental building block for many higher-level protocols, it is crucial for network communication, making it a common protocol in use.

In contrast, SSH, HTTPS, and NFS are all specific application-layer protocols. SSH (Secure Shell) is used for secure remote login and other secure network services, HTTPS (Hypertext Transfer Protocol Secure) is used for secure communication over a computer network, especially on the internet, and NFS (Network File System) allows file sharing across networks. While all of these are indeed common protocols used for specific purposes, TCP is a general transport protocol rather than a specialized application protocol, which aligns it with the core functionality of networking rather than particular applications.

Therefore, identifying TCP as not being a common protocol in the same category as SSH, HTTPS, and NFS reflects this distinction in the layer of the networking stack in which they function.