Explanation: In a regulated power supply, a **Zener Diode** is the most common and effective component for establishing a stable reference voltage. When reverse-biased above its Zener voltage, it maintains a nearly constant voltage drop across it, regardless of significant changes in current. This predictable and stable voltage serves as the crucial reference point against which the power supply's output voltage is compared, allowing the regulator to adjust itself to maintain a steady output. * **A) Tunnel Diode:** Primarily used in very high-frequency applications like oscillators and switches due to its negative resistance characteristic, not for voltage reference. * **B) Battery:** While providing a stable voltage, a battery is a power source itself and would deplete. It's not an active component within the *regulation circuit* designed for a permanent internal reference. * **C) Pass Transistor:** This transistor acts as a variable resistor in series with the load, controlling the output voltage based on the error amplifier's input. It *regulates* the voltage but does not *establish* the reference itself; it works *from* the reference voltage.