Which of the following is a primary voltage regulator?

A Zener diode functions effectively as a primary voltage regulator due to its ability to maintain a constant output voltage despite variations in input voltage or load conditions. When reverse-biased, a Zener diode operates in a breakdown region where it can regulate voltage with remarkable precision. This makes it particularly useful in low-power applications where a stable voltage supply is essential for the proper operation of electronic circuits.

The Zener diode achieves regulation by allowing current to flow only when the input voltage exceeds its Zener voltage, thereby clamping the voltage to this predetermined level. As a result, any excess voltage is dropped across the Zener, keeping the output stable. This characteristic is crucial in applications such as power supplies, reference voltage generation, and signal conditioning.

The other choices have distinct roles. A capacitor primarily acts as a storage device for electrical charge and is used for smoothing out voltage levels, not regulating them. A transistor can serve in voltage regulation circuits, particularly in conjunction with feedback mechanisms, but on its own, it is not classified as a primary voltage regulator. An inductor stores energy in a magnetic field and is utilized in filtering applications, while it can contribute to voltage regulation in complex circuits, it does not serve as a primary regulator by itself.