In the depletion region of an unbiased p-n junction diode there are

immobile ions

Correct answer: immobile ions

Explanation:

In the depletion region of an unbiased p-n junction diode, free charge carriers (electrons and holes) recombine near the junction, leaving behind charged donor and acceptor ions. These ions are immobile because they are fixed in the crystal lattice and cannot move.

Hence, the depletion region contains immobile ions only.