In which process do ionic compounds separate into their oppositely charged constituent ions when they are dissolved in water?
(a) dissociation
(b) electrolyte
(c) hydration
(d) ionization

(A) dissociation

Explanation:

Dissociation in chemistry is a general process in which ionic compounds (such as salts) separate or split into smaller oppositely charged ion

While, an electrolyte is a solution that contains ions and conduct electricity.

on the other hand, Hydration is the process in which a molecule or compound is chemically  attached to water molecules.

But, ionization is the process in which a molecule acquired positive or negative charge on it.