Which of the following is NOT a remote desktop protocol commonly used in Linux?

Remote Desktop Protocol (RDP) is a protocol developed by Microsoft for remote management and remote desktop access on Windows operating systems. While RDP is indeed widely used in Windows environments, it is not a protocol commonly utilized in Linux systems.

On the other hand, VNC (Virtual Network Computing), NX, and Spice are all remote desktop protocols that have established usage within Linux environments. VNC provides a graphical desktop sharing system to remotely control another computer. NX offers an optimized setup that allows for better performance in low-bandwidth situations. Spice is primarily designed for virtual environments and facilitates a seamless connection to virtualized desktop instances.

The distinction lies in the fact that RDP is heavily associated with Windows systems, whereas the other mentioned protocols remain more relevant to Linux and Unix-like systems, making it clear why RDP is the correct answer as the protocol that is not commonly used in Linux.