Ultrasonic motion detectors send out an inaudible tone, capable of __________.

The correct answer is that ultrasonic motion detectors send out an inaudible tone capable of bouncing off walls. Ultrasonic motion detectors operate on the principle of emitting ultrasonic sound waves, which are typically at frequencies above the range of human hearing (above 20 kHz). When these sound waves encounter objects in their path, including walls, they reflect back to the sensor.

This ability to bounce off walls and other surfaces is critical for the functionality of ultrasonic detectors, as it allows them to detect motion based on changes in the frequency of the returning sound waves, known as the Doppler effect. The more objects there are in the vicinity, particularly walls, the better the sensor can function, effectively detecting movement in a defined area.

While measuring distance is a function some ultrasonic devices can perform, the primary and essential capability of the motion detector is its ability to detect motion through reflections off surrounding surfaces. The remaining options, such as detecting heat or reflecting sunlight, do not relate to the operation of ultrasonic motion detectors and thus are not applicable here.