Where is Zener diode used in?
A. Amplification
B. Rectification
C. Oscillator in producing oscillations
D. Voltage regulation

Hint: First let us see where Zener diode is-
Basically, the Zener diode is like an ordinary PN junction diode, but typically works in reverse biased mode. A Zener diode is a heavily doped PN junction diode which has been specially developed.
The depletion layer becomes broader when a PN junction diode is reverse biased. The depletion layer gets progressively as this reverse biased voltage through the diode is constantly increased. At the same time, a steady reverse saturation current due to minority carriers would be present. In the depletion layer, big free electrons are generated because of commutative phenomenon, and the whole diode can become conductive. The process of depletion layer breakdown is known as avalanche breakdown, but it is not quite a sharp breakdown. Compared to avalanche breakdown, there is another form of depletion layer breakdown that is sharper, and this is called Zener breakdown.

Complete step by step solution:
Now let us see the option A which is amplification.
Amplification: The method of raising, without altering any other quality, the magnitude of a variable quantity, especially the magnitude of voltage, power or current. Amplification is done with the help of transistors.
Now let us see what rectification is-

Rectification: Rectification is the process of conversion of signals from A.C to D.C. It is done with the help of diodes only.

Oscillator in producing oscillations- An oscillator is a circuit which without any feedback generates a continuous, repeated, alternating waveform. An oscillator consists of an inductor, a capacitor, amplifier.

Voltage regulator- A voltage regulator is a device programmed to maintain a steady voltage level automatically. In order to retain a steady DC output voltage through the load through fluctuations in the input voltage or changes in the load current, a voltage regulator circuit can be constructed using a Zener diode.

Hence, from the above discussions we can see that option D is correct.

Note: Although diodes are used in amplification, rectification, oscillator, Zener diode is used specifically in voltage regulation. As shunt voltage regulators, Zener diodes are commonly used to control the voltage through small loads.