b
When a \(p - n\) junction is formed, some of the free electrons in the \(n\) region diffuses across the junction and combine with holes to form negative ions. In doing so they leave behind positive ions at the donor impurity sites. Similarly, holes from \(p\) side diffuse to the \(n\) side and thus form a layer called diffusion layer at the junction.So the depletion region is caused by the diffusion of charge carriers.