Which of the following is used as a voltage regulator?a.Photodiodeb.LEDc.Solar celld.Zener diode

The correct answer is d. Zener diode.

Here's why:

A Zener diode is a unique type of diode that allows current to flow not only from its anode to its cathode, but also in the reverse direction when the Zener voltage is reached.

Zener diodes have a highly doped p-n junction. Normal diodes will also break down with a reverse voltage but the voltage and sharpness of the knee are not as well defined as for a Zener diode. Zener diodes are widely used as voltage references and as shunt regulators to regulate the voltage across small circuits.

When connected in parallel with a variable voltage source so that it is reverse biased, a Zener diode conducts when the voltage reaches the diode's reverse breakdown voltage. From that point on, the relatively low impedance of the diode keeps the voltage across the diode at that value.

In simpler terms, a Zener diode, when reverse biased, will keep the voltage across it constant (at its Zener voltage) even if the current through it changes–a handy feature for voltage regulation.

So, Zener diodes are used for voltage regulation.