Answer:
a. Transistor
Step-by-step explanation:
A transistor is an electrical element that can perform two distinct tasks.
It can function either as an amplifier or as a relay/switch: when it operates as an amplifier, it requires a small electrical current at the start (an initial current) and at the other end generates a much larger electrical current (an exit current).
Only when a voltage is applied to the core, a transistor conducts current across the collector-emitter route. If there is no core voltage, the switch will be off. The switch will be on when the core voltage is active.