Which protocol uses an agent on devices to report metrics for network monitoring and troubleshooting?

SNMP (Simple Network Management Protocol) is the correct answer. It uses agents, which are software components running on network devices, to collect and report management information and operational data. A network management system (NMS) then queries these agents to monitor device status, performance, and configuration, aiding in troubleshooting. FIP (Fibre Channel over IP) is a protocol for encapsulating Fibre Channel frames within IP packets, not for network monitoring. NIP (Network Information Protocol) is not a standard networking protocol. NFS (Network File System) is a distributed file system protocol that allows users to access files over a network as if they were local, and is unrelated to network monitoring agents.