Which protocol constantly runs an agent that reports information back to a manager?

The correct choice is SNMP, which stands for Simple Network Management Protocol. SNMP is specifically designed for managing devices on IP networks. It operates in a client-server model where agents (software programs running on network devices) continually gather information about the device’s performance, status, and metrics. This data is then sent back to a central management system known as an SNMP manager.

SNMP operates on a polling mechanism whereby the manager can request data from the agents, but it also supports traps, which allow agents to send information immediately to the manager when certain thresholds are met or events occur. This capability of constant reporting and feedback helps network administrators monitor and manage large networks effectively.

Other choices involve protocols with different functionalities. For instance, WMI (Windows Management Instrumentation) is primarily used within the Windows operating system for managing resources and does not operate as a constant reporting agent in the same way as SNMP. SMTP (Simple Mail Transfer Protocol) is used for sending emails, and it does not involve constant reporting of device status. SMS (Short Message Service) is a standard for text messaging and is unrelated to device management protocols in the context of network monitoring.