Which one of following diode is reverse biased ?

Explanation: 

Diode: A diode is a two-terminal electronic component that conducts current primarily in one direction; it has low resistance in one direction, and high resistance in the other.

Reverse Biasing: ​In reversed bias, the negative region is connected to the positive terminal of the battery and the positive region is connected to the negative terminal. The reverse potential increases the strength of the potential barrier. The potential barrier resists the flow of charge carrier across the junction. It creates a high resistive path in which no current flows through the circuit. Hence to make a reverse bias, N side should be more positive than P side.

In only option (4) N is more positive than P. Hence option (4) is the correct answer.

Additional Information

Forward Biasing: In forward biasing the external voltage is applied across the PN-junction diode. This voltage cancels the potential barrier and provides the low resistance path to the flow of current. The forward bias means the positive region is connected to the p-terminal of the supply and the negative region is connected to the n-terminal of the supply.

•  Very few amount of voltage is required for the complete elimination of the barrier. The complete elimination of the barrier constitutes the low resistance path for the flow of current. Thus, the current starts flowing through the junction. This current is called forward current.
• In forward biasing, the applied voltage V of the battery mostly drops across the depletion region and the voltage drops across the p-side and n-side of the p-n junction is negligibly small.
• It is due to the fact that the resistance of the depletion region is very high as it has no free charge carriers.
• In forward biasing the forward voltage opposes the potential barrier Vb. As a result of it, the potential barrier height is reduced and the width of the depletion layer decreases. Therefore option 2 is correct.
• As forward voltage is increased, at a particular value the depletion region becomes very much narrow such that a large number of majority charge carriers can cross the junction.