What types of initiating devices are commonly used in fire alarm systems?

The correct answer highlights the types of initiating devices that are specifically designed and recognized for use in fire alarm systems. Smoke detectors play a crucial role in detecting the presence of smoke, which is often an early indicator of fire. Heat detectors serve another vital function by sensing rapid increases in temperature, which can signify a fire's presence. Manual pull stations empower occupants to report a fire emergency directly by pulling a lever, activating the alarm system, and alerting others to evacuate.

In contrast, the other options include devices that are not primarily designed for fire detection. For example, video cameras and infrared sensors may serve security purposes but are not standard initiating devices in fire alarm systems. Similarly, light sensors and motion detectors are typically used for security and occupancy detection rather than fire safety. Lastly, thermostats, humidity gauges, and pressure sensors are used for environmental monitoring in HVAC systems, not for detecting fires. This underscores the unique role of smoke detectors, heat detectors, and manual pull stations within fire alarm systems, emphasizing why they are classified as initiating devices critical for fire detection and response.