Which of the following items is classified as a power source?

A solar cell is classified as a power source because it directly converts sunlight into electrical energy through the photovoltaic effect. This conversion process allows it to generate a usable electric current, making it a key component in renewable energy systems and a common example of a power source utilized in various applications, such as solar panels for residential energy supply.

In contrast, a relay functions as an electrically operated switch and does not produce power; it merely controls circuits. An inductor is a passive electrical component that stores energy in a magnetic field when electric current flows through it, but it also does not generate power. Similarly, a transistor is a semiconductor device that can amplify or switch electronic signals but is not a power source itself. Thus, among the options listed, the solar cell stands out as the only item that actively generates electrical power.