Which of the following can provide power gain?

A transistor is an active semiconductor device capable of amplifying an electrical signal, which means it can provide power gain. By using a small input current or voltage to control a much larger current from a power supply, it effectively multiplies the power of the original signal. Other choices like transformers, reactors (inductors), and resistors are passive components. While a transformer can step up voltage or current, it cannot increase the total power; in fact, there is always a slight power loss due to inefficiency. Resistors only dissipate power.