Final answer:
A ping sweep is the technique used to determine which hosts are live on a network by sending ICMP echo requests to multiple IP addresses and listening for replies.
Step-by-step explanation:
To determine which hosts are live on a network, the correct technique to use is a ping sweep. This method involves sending Internet Control Message Protocol (ICMP) echo requests, commonly known as pings, to a range of IP addresses. Any IP address that responds with an ICMP echo reply is considered a live host. A ping sweep can be performed using various tools and is generally used by network administrators to troubleshoot or map a network. Other options listed, such as a Smurf attack, Fraggle attack, and session hijacking, are types of network attacks and are neither ethical nor legal methods for checking live hosts.