For confined interior areas containing explosive vapors or materials, non-emitting sensors are preferred to avoid ignition risk. Passive Infrared (PIR) sensors are ideal in such environments because they detect heat signatures (infrared energy) emitted by intruders without emitting any signals or energy that could trigger an explosion.
A (Ultrasonic) and C (Capacitance) sensors emit signals.
D (Passive vibration sensors) detect vibration and are used in a different context, like surface tampering.
References:
PSP Study Guide - Intrusion Detection Systems and Safety Considerations POA Manual - Sensor Selection for Hazardous Environments