Which sensor is unaffected by external noise and can detect air movement caused by fire?

The ultrasonic sensor is the correct choice because it operates by emitting sound waves at frequencies higher than the audible range for humans. When these sound waves encounter objects, such as the movement of air caused by a fire, they reflect back to the sensor. This technology allows the ultrasonic sensor to effectively detect changes in the environment, including air movement, regardless of external noise that may interfere with other sensing technologies.

Additionally, ultrasonic sensors are particularly sensitive to air currents and can provide an early warning in fire detection scenarios, making them highly suitable for applications where monitoring air flow is critical. Their ability to function in noisy environments further enhances their effectiveness compared to other sensors in the options presented.