To solve this problem, we need to understand the behavior of the depletion region in a p-n junction diode under different biasing conditions.$\\$The depletion region is a region around the p-n junction where mobile charge carriers are depleted.$\\$This region is formed due to the diffusion of electrons and holes across the junction, leading to the formation of a built-in potential.$\\$Let's analyze the effect of different biasing conditions:$\\$• Forward Bias:$\\$In forward bias, the p-side is connected to the positive terminal and the n-side to the negative terminal of a battery.$\\$This reduces the built-in potential barrier, allowing more charge carriers to cross the junction.$\\$As a result, the width of the depletion region decreases.$\\$• Reverse Bias:$\\$In reverse bias, the p-side is connected to the negative terminal and the n-side to the positive terminal of a battery.$\\$This increases the potential barrier, preventing charge carriers from crossing the junction.$\\$Consequently, the width of the depletion region increases as more immobile ions are exposed.$\\$Based on this analysis, the increase in the width of the depletion region is due to reverse bias only.$\\$Therefore, the correct option is Option 1: Reverse bias only.