Which of the following protocols would be used to test if one host is reachable by another host, by sending echo requests and waiting for echo replies?

SNMP (Simple Network Management Protocol) is used for monitoring and managing network devices, not for basic host reachability testing via echo requests.

The Internet Control Message Protocol (ICMP) is specifically designed to send control and error messages, including echo request and echo reply messages. These messages are fundamental to network utilities like `ping`, which use them to test the reachability and round-trip time between two IP hosts.

TCP (Transmission Control Protocol) is a connection-oriented protocol for reliable data transfer and does not inherently use echo requests/replies for host reachability testing.

UDP (User Datagram Protocol) is a connectionless protocol for fast, unreliable data transfer, and it does not define echo requests or replies for reachability testing.