Which of the following can provide power gain?

A transistor can provide power gain by amplifying an input signal. It works as a semiconductor device that controls the flow of electrical current, allowing a small input signal at its base to control a much larger output current flowing from the collector to the emitter. This characteristic is fundamental to building amplifiers, where the goal is to increase the power level of a signal without altering its original properties, like frequency or waveform.

In contrast, a transformer primarily functions to change the voltage levels while conserving power; it does not amplify power. A reactor, typically an inductor, stores energy but does not provide power gain either. A resistor dissipates energy in the form of heat and likewise does not contribute to any power gain. Thus, among the options, the transistor is the only device specifically designed to achieve power amplification.