Final answer:
Port 3389 must be open for Remote Desktop Protocol (RDP) to function. This is the default port used for establishing a connection to a remote Windows computer.
Step-by-step explanation:
The port that MUST be open in order for Remote Desktop to function is C. 3389. Remote Desktop Protocol (RDP) uses port 3389 by default to connect to and control a remote computer running Windows operating systems. This port must be forwarded properly in the firewall to allow an RDP connection.
Other ports mentioned like A. 53, which is used for DNS services, B. 80, which is the standard port for HTTP, or D. 5900, commonly used for VNC (another remote desktop software), are not required for the standard operation of Microsoft Remote Desktop.