Telnet is a protocol used for accessing remote computers over a TCP/IP network, and it runs over TCP port 23.
Telnet Protocol: Telnet allows remote command-line interface access to network devices and servers.
Port 23: The standard port for Telnet is TCP port 23.
Option D is correct as Telnet operates over TCP port 23.
Reference:
Cisco DevNet Documentation: Telnet Protocol and Port Numbers