In the depletion region of a diode:
(a) There are no mobile charges.
(b) An equal number of holes and electrons exist, making the region neutral.
(c) Recombination of holes and electrons has taken place.
(d) Immobile charged ions exist.

Choose the correct option from the given ones:
1. (a), (c), (d) 2. (b), (d)
3. (a), (d) 4. (b), (c)