Heavily doped p-n junction diode which emits light when it is forward biased is/are
A. zener diode
B. photodiode
C. light emitting diode
D. all of the above

Hint: A diode can be defined as an electric device which carries current only in one direction. It has zero resistance ideally. A diode is created by applying p-type silicon on a plate made of n-type silicon. When this application or doping is heavy, certain types of diodes produce light, which can be utilized further.

Complete answer:
The correct answer for this question is option C because light emitting diodes are usually made of highly- thin layers of heavily doped semiconductor materials. When these diodes are forward biased, a colored light at a specific spectral wavelength is emitted, which usually depends on the semiconductor material used as well as the amount of doping. This happens because electrons from the conduction band of the semiconductor recombines with the holes from the valence band of the semiconductor. Also, when the diode is forward biased, an ample amount of energy is released, which is sufficient enough to produce photons, which will emit a monochromatic light of single color. Due to their thin layers, a good number of photons can leave the junction and produce a colored light output during radiation.

Therefore, the correct answer is option C.

Additional Information:
Light Emitting Diodes are usually called LEDs. These devices use semiconductors to produce light while passing current through them. LED bulbs use electricity very efficiently. Light emitting diodes do not emit a lot of heat as they are integrated with heat sinks, which absorb most of the heat. Light emitting diodes convert electric energy directly into light energy, which results in efficient light generation, with least electricity wastage.

Note:
Light emitting diodes (LEDs) are energy efficient devices which generate more light using less electricity, when compared to conventional bulbs. They are highly efficient and long-lasting devices. They do not overheat due to regular use for a long time and hence have very slight chances of overheating or catching fire.