How can a client identify which hosts NPM is running on in Datadog?

The correct answer is based on the specific capabilities of the metrics associated with Datadog’s Network Performance Monitoring (NPM). Using the metric datadog.system_probe.agent is the appropriate method to identify which hosts NPM is running on.

This metric provides insights into the agents that are installed and actively monitoring the network performance on various hosts. It effectively allows clients to track the status and performance metrics related to network monitoring, thereby enabling them to determine which instances of NPM are currently active and providing telemetry data.

In contrast, the other metrics mentioned do not serve the purpose of identifying active NPM hosts as directly or effectively. Each of these metrics pertains to different aspects of Datadog's functionalities but does not specifically concentrate on the active deployment of the NPM agent, which is why using the system probe agent metric is the most suitable choice for this task.