When you walk through a metal detector carrying a metal object in your pocket, it raises an alarm. This phenomenon works on

A metal detector raises an alarm when a metal object passes through it. We need to identify the underlying principle.

Understand how a metal detector works.

A metal detector uses a coil that is part of an AC circuit tuned to a specific resonant frequency. When a metallic object comes near the coil, it changes the inductance and impedance of the circuit, shifting the resonant frequency and altering the current amplitude.

Identify the correct principle.

The detector works by monitoring the resonance condition of the AC circuit. When metal enters the field, the change in impedance disturbs the resonance, which triggers the alarm. This is the principle of resonance in AC circuits.

The correct answer is Option B: Resonance in ac circuits.