What is true about the breakdown voltage in a Zener diode?
A) It decreases when current increases
B) It destroys the diode
C) It equals the current times the resistance
D) It is approximately constant

Hint : A Zener diode is often used as a voltage regulator since it only allows current to flow in one direction. When a Zener diode allows current to flow in the circuit, the voltage across it remains constant but the current increases drastically.

Complete step by step answer
When a PN junction diode is highly doped with impurities, the higher concentration of ions in the depletion layer causes the width of the depletion layer to become thinner than that in a normally doped PN junction diode. Due to this thinner depletion layer, the voltage drop across the depletion layer is quite high.
If a reverse voltage is applied to the Zener diode and is continued to increase, after a certain applied voltage, the electrons within the depletion region come out and make the depletion region conductive. This is called Zener breakdown. At this point, the voltage across the Zener diode remains nearly constant but the current increases drastically since the path of current is conductive.
The voltage point at which the voltage across the Zener diode allows current to flow in reverse bias is called the Zener voltage. However, this Zener voltage remains constant once breakdown occurs and only the current flowing will increase beyond the Zener voltage.
Hence option (D) is correct.

Note
The breakdown voltage is only approximately constant for voltages beyond the Zener voltage. For normal Zener diodes, this voltage can range from less than one volt to a few hundred volts. Due to its property of regulating the voltage, it is used as a voltage regulator since it won't allow current to flow below Zener voltage and above Zener voltage, it will maintain a constant potential difference across it.