Which of the following protocols operates on port 53?

The correct answer is DNS, which stands for Domain Name System. This protocol is used for translating human-readable domain names into IP addresses, which are necessary for locating and identifying computer services and devices with the underlying network protocols. Port 53 is designated for DNS queries and responses, allowing clients to query DNS servers to resolve domain names into their corresponding IP addresses.

The other options are associated with different functions and utilize different ports: HTTP operates on port 80, primarily used for web traffic; FTP, or File Transfer Protocol, utilizes port 21 for transferring files; and Telnet, a protocol for text-based communication, operates on port 23. Each protocol has its specific duties within the network, defined by the ports allocated by the Internet Assigned Numbers Authority (IANA).