What would a PIR most effectively detect?

A Passive Infrared Sensor (PIR) is specifically designed to detect motion based on changes in infrared radiation in its environment. It is most effective at identifying the movement of objects that emit heat, such as people and animals. When it comes to its primary function, a PIR sensor detects motion in a designated area, which typically includes tracking the movement of individuals as they traverse within that space.

The correct choice emphasizes the PIR's primary application in detecting movement rather than stationary objects or passive changes to the environment. For example, while a PIR could potentially detect large animals if they trigger a change in thermal activity, its high sensitivity to smaller, heat-emitting bodies like people makes it particularly adept at monitoring areas like crosswalks, where pedestrian movement is a critical concern.

In contrast, choices such as open doors or breaking windows involve changes that are not directly related to the radiant heat emitted by living beings in motion, which fall outside the optimal capabilities of a PIR. These other scenarios might require different detection systems, such as door/window contacts or acoustic sensors, which are better suited for those specific types of events. Thus, a PIR is not primarily intended for detecting the state of doors or the sound associated with the breaking of glass.