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

The correct answer is Option (2) → immobile ions

When the junction forms, electrons diffuse from the n-side into the p-side and holes from the p-side into the n-side, and they recombine. Near the junction this leaves behind the fixed ionised donor atoms (positive, on the n-side) and fixed ionised acceptor atoms (negative, on the p-side). The region is depleted of free charge carriers and contains only these ions, which are bound in the crystal lattice.

NCERT (2026-27):
p-n Junction Formation – “… When an electron diffuses from n → p, it leaves behind an ionised donor on n-side. This ionised donor (positive charge) is immobile as it is bonded to the surrounding atoms. … This space-charge region on either side of the junction together is known as depletion region as the electrons and holes taking part in the initial movement across the junction depleted the region of its free charges (Fig. 14.10).”