What type of detector monitors the environment and trips when the radiant heat energy signature of an intruder is detected?

The correct choice is a detector that relies on the detection of infrared radiation to identify the presence of intruders based on their body heat. Passive infrared detectors operate by detecting the infrared radiation emitted by objects in their environment, specifically focusing on the heat signatures of living beings like humans or animals.

These detectors are designed to be sensitive to changes in heat signatures, where the sensor becomes activated when a warm body enters the field of view. Unlike other types of detectors, passive infrared sensors do not emit any energy of their own; they simply respond to the infrared energy radiating from the intruder.

This capability makes passive infrared detectors particularly effective in security applications, as they can distinguish between the heat from a human body and the surrounding environment, reducing false alarms caused by non-living heat sources or changes in temperature.