Which of the following protocols is used to transmit ARP requests and replies?

The correct answer is ARP, which stands for Address Resolution Protocol. ARP is specifically designed to map IP addresses to MAC addresses within a local network. When a device wants to communicate with another device over a network, but only knows the target's IP address, it utilizes ARP to broadcast a request asking "Who has this IP address?" The device that holds the corresponding IP address replies with its MAC address, allowing for proper data transmission at Layer 2 of the OSI model.

Understanding this function highlights why ARP is the only appropriate choice for this question. The other protocols listed serve different purposes: ICMP (Internet Control Message Protocol) is primarily used for sending error messages and operational information; IPSec is a suite of protocols used for securing Internet Protocol (IP) communications; and UDP (User Datagram Protocol) is a transport layer protocol used for communication between applications. Each of these protocols operates in different contexts and for different types of data transmission, making ARP uniquely suited for handling requests and replies regarding IP address resolution.