Zener breakdown occurs in a p - n junction having p and n both :

CONCEPT:

Zener diode - The Zener diode is defined as the semiconductor device which permits the current to flow in the forward or backward directions. It is a heavily doped p - n junction and also it is designed to conduct in the reverse direction when a certain specified voltage is reached in the given circuit. It is having a well-defined reverse-breakdown voltage, at which it starts conducting current, and continues operating continuously in the reverse-bias mode without getting damaged. In addition to it, the voltage drop across the diode remains constant over a wide range of voltages, a feature that makes Zener diodes suitable for use in voltage regulation.

EXPLANATION:

As we know that the Zener diode is a semiconductor device that permitted the current to flow in the forward or backward direction. The Zener breakdown occurs in the heavily doped p-n junction diode. Heavily doped p-n junction diodes have a narrow depletion region and this narrow depletion layer width leads to a high electric field which causes the p-n junction breakdown.

Therefore Zener breakdown occurs in a p - n junction having p and n both heavily doped and narrow depletion layers. 

Hence, Option 3) is the correct answer.