What is the name of the automatic fire suppression device that operates when its heat-activated element reaches a certain temperature?

The automatic fire suppression device referred to in the question is known as an automatic sprinkler. These devices are designed to extinguish or control fires by discharging water when their heat-activated element, typically a glass bulb or fusible link, reaches a specific temperature. This activation mechanism allows sprinklers to react to elevated heat levels indicative of a fire, providing a timely and effective response.

In contrast, a fire alarm system is intended for detection and signaling purposes, alerting occupants and emergency services rather than suppressing fire. A smoke detector detects smoke, which indicates potential fire, but it does not extinguish fires. Finally, a water spray system is a form of fire protection but is employed differently, usually for specific hazards and not as an automatic response to fire like sprinklers. Thus, the automatic sprinkler is rightly identified as the device that operates based on temperature activation.