When used as a voltage regulator, a Zener diode should be:

Zener diode is called Voltage regulator diode because it maintains a constant voltage across its terminals called Zener voltage once the breakdown happens.

Zener diode: A Semiconductor Diode blocks current in the reverse direction, but will suffer from premature breakdown or damage if the reverse voltage applied across becomes too high.

Zener diode as voltage regulator: The Zener diode is connected with its cathode terminal connected to the positive rail of the DC supply so it is reverse biased and will be operating in its breakdown condition. Resistor Rs is selected so to limit the maximum current flowing in the circuit.

Explanation:

• A Zener diode is used as a voltage regulator in reverse biased mode.
• The breakdown voltage in the Zener diode when connected in the reverse-biased is called Zener voltage. This Zener voltage so steady and constant, it has huge applications in circuits, most importantly, voltage regulation.
• Hence the correct option is – Reverse biased