Final answer:
FTP uses TCP/21 as its control port and TCP/20 for data transfer, so the correct answer is c. TCP/21. The other options reflect different protocols or incorrect port assignments.
Step-by-step explanation:
The File Transfer Protocol (FTP) is a standard network protocol used to transfer files from one host to another over a TCP-based network, such as the Internet. FTP uses two ports for its operations - one for commands and control (control port), and one for sending the actual data (data port).
TCP/21 is the standard control port used by FTP for sending commands, while TCP/20 is used for the data transfer. Therefore, the correct option from the list given in this question for the FTP control port is c. TCP/21.
Options a, b, d, e, and f describe other services or incorrect combinations. For example, UDP/69 is used by TFTP (Trivial File Transfer Protocol), which is a different protocol.