What protocol allows users to remotely control another computer through a graphical interface and operates on port 3389?

The correct choice is Remote Desktop Protocol (RDP). RDP is specifically designed for remote management and access to a computer's desktop interface, allowing users to connect to a remote computer and control it as though they were sitting right in front of it. This protocol operates over port 3389, which is standardized for RDP traffic, ensuring that the communication between the client and server is established using this specific port.

When using RDP, users benefit from a graphical interface, making the experience more intuitive compared to text-based remote access methods. This graphical environment allows for easier navigation and management of applications on the remote machine, which is particularly useful for system administration and technical support purposes.

The other protocols described do not serve this remote control function. File Transfer Protocol (FTP) is used primarily for transferring files between computers. Hypertext Transfer Protocol (HTTP) is used for transmitting web pages on the internet. Secure Copy Protocol (SCP) is utilized for securely transferring files between hosts on a network. None of these protocols offer the same kind of interactive remote desktop functionality that RDP provides.