Final answer:
The protocol that operates on ports 20 and 21 and is commonly attacked by hackers is the File Transfer Protocol (FTP).
Step-by-step explanation:
The protocol that operates on ports 20 and 21 and is commonly attacked by hackers is the File Transfer Protocol (FTP). FTP is a standard network protocol used for transferring files from one host to another over a TCP-based network like the internet. It uses ports 20 and 21 to establish the connection and transfer data.