Which of the following is a power source?

The solar cell is correctly identified as a power source because it converts sunlight directly into electrical energy through the photovoltaic effect. This process involves the generation of direct current when light photons strike the cell material, typically silicon, causing electrons to be knocked loose and generating an electrical current.

In contrast, a relay is an electromechanical switch that is used to control a circuit but does not generate power. An inductor stores energy in a magnetic field when electrical current flows through it, but it doesn’t produce power on its own; rather, it is a passive component used in electrical circuits. A transistor is a semiconductor device that can amplify or switch electronic signals, but it also does not serve as a power source by itself.

Thus, the solar cell stands out as the only option that inherently generates power from an external energy source, specifically sunlight.