How is a waterflow alarm activated?

A waterflow alarm is activated by the flow of water through the fire protection system when a sprinkler head opens, which is why this answer is the correct choice. When a sprinkler head activates due to heat from a fire, it allows water to flow from the system. This movement of water is detected by flow switches that are installed in the piping system. The flow switches are designed to respond specifically to the presence of water flowing through the pipes, triggering the alarm and alerting occupants and authorities to a potential fire condition.

In contrast, the other options describe different mechanisms that do not apply to waterflow alarm activation. A change in air pressure does not directly indicate water flow and is typically related to other system monitoring aspects. Similarly, a smoke detection signal relates to fire detection through sensing smoke, not water flow. Finally, while manual operation by an inspector could influence system testing or maintenance, it does not directly activate a water flow alarm, which is designed to respond automatically to fire conditions. Therefore, the mechanism for activating a waterflow alarm relies entirely on the flow of water resulting from sprinkler head activation.