The correct answer is option 'D', which states that the increase in the width of the depletion region in a p-n junction diode is due to reverse bias only. Let's understand this in detail:

Introduction to p-n Junction Diode:
A p-n junction diode is formed by joining a p-type semiconductor (excess holes) with an n-type semiconductor (excess electrons). The region near the junction where the majority carriers (electrons in n-type and holes in p-type) are depleted is called the depletion region.

Depletion Region:
The depletion region is a region that lacks any free majority carriers, resulting in a region with a fixed charge. The width of the depletion region is the region where the electric field associated with the fixed charge is significant.

Forward Bias:
When a p-n junction diode is forward biased, the positive terminal of the battery is connected to the p-side and the negative terminal to the n-side. This causes the majority carriers to move towards the junction, resulting in a reduction in the width of the depletion region. The electric field across the junction weakens, allowing current to flow through the diode.

Reverse Bias:
When a p-n junction diode is reverse biased, the positive terminal of the battery is connected to the n-side and the negative terminal to the p-side. This causes the majority carriers to move away from the junction, increasing the width of the depletion region. The electric field across the junction strengthens, preventing current flow through the diode.

Explanation of the Correct Answer:
The increase in the width of the depletion region in a p-n junction diode is due to reverse bias only. When a reverse bias is applied, the majority carriers (electrons in n-side and holes in p-side) are pushed away from the junction, widening the depletion region. This occurs because the positive terminal of the battery repels the majority carriers in the n-side, and the negative terminal repels the majority carriers in the p-side. As a result, the width of the depletion region increases.

In forward bias, the width of the depletion region decreases because the majority carriers are attracted towards the junction. The electric field across the junction weakens, allowing current to flow. Therefore, the increase in the width of the depletion region is not due to forward bias.

To summarize, the increase in the width of the depletion region in a p-n junction diode is solely due to reverse bias.