In forward biasing of a p-n junction,

The correct answer is Option (4) → the positive terminal of the battery is connected to the p-side and the depletion region becomes thin

Option (1) the positive terminal of the battery is connected to the p-side and the depletion region becomes thick — Incorrect. The connection is right. But the applied voltage opposes the built-in potential, so the depletion region becomes thinner.

Option (2) the positive terminal of the battery is connected to the n-side and the depletion region becomes thin — Incorrect. Positive terminal to the n-side is reverse bias. Under reverse bias, the depletion region widens.

Option (3) the positive terminal of the battery is connected to the n-side and the depletion region becomes thick — Incorrect. This describes a reverse-biased junction, where the applied voltage adds to the barrier.

Option (4) the positive terminal of the battery is connected to the p-side and the depletion region becomes thin — Correct. In forward bias the p-side goes to the positive terminal and the n-side to the negative terminal. The applied voltage \( V \) is opposite to the built-in potential \( V_0 \), so the depletion layer width decreases and the barrier height falls to \( V_0-V \).

NCERT (2026-27):
p-n Junction Diode under Forward Bias – “When an external voltage V is applied across a semiconductor diode such that p-side is connected to the positive terminal of the battery and n-side to the negative terminal …, it is said to be forward biased.”

p-n Junction Diode under Forward Bias – “… The direction of the applied voltage (V) is opposite to the built-in potential V₀. As a result, the depletion layer width decreases and the barrier height is reduced …. The effective barrier height under forward bias is (V₀ – V).”