Which of these components can be used as an electronic switch?

Transistors are fundamental components in electronics and are widely recognized for their ability to act as electronic switches. When a small input voltage or current is applied to the base terminal of a transistor, it can control a larger current flowing between the collector and emitter terminals. This property makes transistors highly effective for switching applications, enabling them to turn circuits on and off or to amplify signals.

In contrast, varistors, potentiometers, and thermistors serve different primary functions. A varistor is mainly used for voltage regulation and protection against voltage spikes. A potentiometer is a variable resistor, primarily used for adjusting levels of voltage or current rather than switching them on and off. A thermistor is a temperature-sensitive resistor that changes its resistance with temperature variations, and while it can be part of a temperature control circuit, it is not typically used as a switch in the way a transistor is. This clear distinction in functionality highlights why the transistor is the correct choice for an electronic switch.