Applying forward bias to p-n junction, the potential barrier

A forward-biased p-n junction has an external voltage applied with positive terminal to the p-side and negative terminal to the n-side, opposing the built-in potential barrier across the depletion region.

This external electric field reduces the effective potential barrier height, allowing majority carriers (holes from p-side, electrons from n-side) to cross the junction more readily, resulting in significant current flow.

Therefore, forward biasing a p-n junction decreases the potential barrier.

Final answer: $\boxed{B}$