What device is typically used as a stable reference voltage in a linear voltage regulator?

In voltage regulator circuits, a Zener diode is specifically designed to operate continuously in its reverse-breakdown region to provide a highly stable reference voltage. When a reverse bias exceeds its Zener voltage rating, it holds that voltage constant across its terminals regardless of minor current fluctuations. The other components listed serve entirely different purposes in a circuit. A standard junction diode is used for rectification, a varactor diode acts as a voltage-variable capacitor for tuning circuits, and an SCR (Silicon Controlled Rectifier) is used for switching higher power loads. Therefore, only the Zener diode is appropriate for a voltage reference.