A vendor requires remote access to a specific server to maintain a proprietary software system. Security policy restricts the use of port 3389 to employees but does allow command-line access to…

RDP provides a complete graphical desktop session to a remote server, which is typically required to install, configure, or maintain proprietary software applications that have a GUI interface. Since the security policy already permits command-line access for non-employees, the additional capability that would specifically need to be enabled for the vendor is RDP on port 3389 to support the interactive, graphical maintenance tasks the proprietary system requires.

Telnet provides unencrypted command-line access and is considered insecure for any production environment, making it unsuitable and unlikely to be approved for vendor access.

SSH provides encrypted command-line access and is already described as permitted for non-employees in the security policy, so it would not need to be additionally enabled for the vendor.

FTP is a file transfer protocol and does not provide remote administration or interactive session capabilities needed to maintain and configure a running software system.