Infrared motion detectors are designed to respond to:

Infrared motion detectors are primarily designed to respond to heat. They operate based on the principle of infrared radiation, which is emitted by all objects with a temperature above absolute zero, including humans and animals. When a person or an object with a different temperature than the surrounding environment moves within the detector's range, it generates a change in infrared energy that the device can detect.

This change is what triggers the alarm or the action associated with the motion detector. The technology is adept at detecting the heat signature of a moving body compared to stationary objects, which is crucial for applications such as security systems in homes and businesses.

While motion is a related concept and can be associated with the operation of these detectors, the underlying principle concerning how they function centers on detecting variations in heat rather than mere motion, making heat the more accurate answer in this context. Other options such as light and sound do not directly pertain to the functionality of infrared motion detectors, which are specifically designed to sense thermal changes.