For a transistor to act as a switch, it must be operated in

We need to determine in which region a transistor must operate to act as a switch.

Understanding transistor as a switch:

A switch has two states — ON and OFF.

OFF state (Cut-off region): When no base current flows (or the base-emitter voltage is below the threshold), both junctions are reverse biased. No collector current flows, and the transistor acts like an open switch.

ON state (Saturation region): When sufficient base current flows, both junctions are forward biased. Maximum collector current flows, and the transistor acts like a closed switch with very low resistance.

The active region is used for amplification, not switching, because in this region the collector current is proportional to the base current (linear behaviour).

Therefore, for a transistor to act as a switch, it must operate in the saturation and cut-off regions — switching between these two states.

The correct answer is Option B.