Explanation:

When a diode is forward-biased, it allows the flow of current through it. However, once the forward current reduces to zero, a practical diode does not immediately stop conducting in the reverse direction. This is due to the presence of stored charges in the depletion region of the diode.

Depletion Region:
The depletion region is a region in a diode where there are no majority charge carriers (electrons in the N-region and holes in the P-region). It forms due to the diffusion of charge carriers across the junction when a diode is not biased. In this region, there are immobile ions and fixed charges.

Stored Charges:
When a diode is forward-biased, the depletion region becomes very narrow, and the majority charge carriers (electrons and holes) cross the junction and contribute to the forward current. However, when the forward current reduces to zero, the depletion region expands again, and the majority charge carriers are no longer present in the region.

However, there are still immobile ions and fixed charges in the depletion region. These charges are responsible for creating an electric field across the junction. When the diode is reverse-biased, the electric field pushes the minority carriers (minority carriers are electrons in P-region and holes in N-region) towards the junction.

Conduction in Reverse Direction:
The stored charges in the depletion region create a potential barrier that needs to be overcome for the diode to conduct in the reverse direction. This potential barrier prevents the flow of current initially. However, due to the thermal energy, some minority carriers acquire enough energy to overcome the potential barrier and cross the junction, resulting in a small reverse current.

This small reverse current is known as the reverse saturation current and is typically very small compared to the forward current. It is also temperature-dependent, increasing with temperature.

Therefore, even after the forward current reduces to zero, a practical diode continues to conduct in the reverse direction for a while due to the stored charges in the depletion region. As the stored charges gradually dissipate, the reverse current decreases, and the diode eventually stops conducting in the reverse direction.

Conclusion:
The correct answer is option C - stored charges in the depletion region. These stored charges create a potential barrier that allows the diode to conduct in the reverse direction even after the forward current reduces to zero.