The cause of the depletion layer in the p-n junction region is

diffusion of charge carriers

The correct answer is Option (4) → diffusion of charge carriers

The depletion layer in a p-n junction is formed because electrons from the n-side diffuse into the p-side and recombine with holes, while holes from the p-side diffuse into the n-side and recombine with electrons. This diffusion leaves behind uncovered immobile ions, creating the depletion region.

Answer: diffusion of charge carriers