Answer: (b) reverse bias
When a PN junction diode is heavily doped with impurities, the depletion layer’s breadth narrows due to the larger concentration of ions in the depletion layer than in a regularly doped PN junction diode. The voltage drop across the depletion layer is relatively large due to the thinner depletion layer. If a reverse voltage is provided to the Zener diode and then increased, the electrons within the depletion zone will come out and make the depletion region conductive after a specific applied voltage. This is referred to as a Zener breakdown. The voltage across the Zener diode remains virtually constant at this moment, but the current increases dramatically since the current route is conductive.
While the Zener diode behaves exactly like a normal PN junction diode when connected in forward bias, its property of a sharp breakdown voltage in reverse bias is why we use it as a voltage regulator because it will only allow current to flow beyond a certain voltage.