A depletion layer consists of

(a) Electrons

(b) Protons

(c) Mobile ions

(d) Immobile ions

(d)

Depletion layer is formed by diffusion of holes and electrons from p-type semiconductor to n-type semiconductor and vice-versa. Hence, donor and acceptor atom get positive and negative charge leading to formation of p-n junction. Thus, donor and acceptor are immobile.