What qualifies as a fire alarm?

A fire alarm is defined as a signal that is triggered by a device indicating the presence of fire. This encompasses various types of fire detection systems, including smoke detectors and heat sensors, which respond to specific conditions indicative of a fire, such as the presence of smoke, heat, or flames. The primary function of a fire alarm system is to provide early warning to occupants of a building, allowing for timely evacuation and response to the emergency.

In contrast, the other options do not meet the criteria for what constitutes a fire alarm. A device that only turns on lights when smoke is detected does not communicate the presence of fire itself; therefore, it cannot be categorized as a complete fire alarm system. Similarly, a system that controls building access during a fire serves a different purpose, focusing on safety and security rather than detection. An early warning system for gas leaks addresses a different hazard altogether and is not designed to detect fire. Thus, option B stands as the definition of a fire alarm due to its role in signaling fire detection.