Explanation:

When a Zener diode is revers biased, it acts like a voltage regulator as in the breakdown region Zener diode has constant voltage for a large change in current. When an unregulated voltage is applied across the circuit shown, the Zener diode provides a constant voltage drop across it. During regulation, the current changes through the series resistance R that, leads to a change in the Zener diode resistance. This leads to a constant voltage output across its terminals and the voltage is regulated.