Which of the following protocols is used to prevent loops in Ethernet networks?

The protocol used to prevent loops in Ethernet networks is the Spanning Tree Protocol (STP). It is specifically designed to handle loop prevention in network topologies that include redundant paths. In Ethernet networks, loops can occur when there are multiple active paths between switches. These loops can lead to broadcast storms, MAC table instability, and network outages, which can severely disrupt the functionality of the network.

STP works by creating a tree-like structure of the network topology and placing certain ports in a blocking state to eliminate redundant paths. This ensures that there is only one active path between any two nodes in the network. If the active path fails, STP can reconfigure the network dynamically by activating a previously blocked path, thus maintaining redundancy without creating a loop.

The other options mentioned serve different purposes: Link Aggregation (LAG) is used for combining multiple network connections for increased bandwidth; ARP (Address Resolution Protocol) is responsible for mapping IP addresses to MAC addresses; and ICMP (Internet Control Message Protocol) is used for error reporting and diagnostic functions in IP networks. These protocols do not address the issue of loops in Ethernet networks, highlighting why STP is the correct choice for this specific function.