The correct answer is: A. Decreases with light doping.
The depletion layer is a region at the junction of a P-type and N-type semiconductor where there are no free charge carriers. The width of the depletion layer depends on the doping concentration of the P-type and N-type semiconductors. The higher the doping concentration, the narrower the depletion layer. This is because the higher doping concentration results in a higher concentration of free charge carriers in the P-type and N-type semiconductors, which reduces the electric field at the junction and hence the width of the depletion layer.
Option B is incorrect because the depletion layer increases with heavy doping.
Option C is incorrect because the width of the depletion layer is not independent of applied voltage. The applied voltage affects the electric field at the junction, which in turn affects the width of the depletion layer.
Option D is incorrect because the depletion layer is decreased under reverse bias.