Which of the following default ports should be open on a server for remote administration?

TCP 25 is the default port for SMTP, used for sending email between mail servers, not for remote administration.

TCP 389 is the default port for LDAP, used for directory services queries and authentication lookups, not remote desktop administration.

TCP 3128 is a common default port for the Squid proxy server, not for remote system administration.

TCP 3389 is the well-known port for RDP (Remote Desktop Protocol), Microsoft's built-in remote administration service for Windows servers and desktops. It allows administrators to connect to and manage remote systems graphically over a network. This port must be open on the server's firewall to allow inbound RDP sessions.