Which of the following is the type of sensor used to protect metal containers by...

The correct answer is Capacitance.

Capacitive sensors detect changes in the electromagnetic field around a metallic object. When a metal container enters this field, it alters the capacitance, triggering an alarm or alert.

Photoelectric sensors rely on light interruption or reflection and are typically used for perimeter detection.

Sonic and ultrasonic sensors measure sound waves to detect motion or proximity but are not designed to sense metallic objects via electromagnetic fields.

Capacitive sensing is widely used in security for protecting safes, metal cabinets, and secured storage containers because it provides reliable detection of tampering or unauthorized entry into a defined magnetic field, consistent with ASIS PSP guidance on electronic intrusion detection.