The process of occurs when a hydrate loses its water of hydration.
a. effloresce
b. hydration
c. deliquescent
d. solvation

Let us understand all the terms

Hydration : this is the addition of water to anything. It can be addition of water to double or triple bond, absorption of water by a substance. so it cannot be loss of water.

Deliquescent : it is the absorption of water by a substance and getting dissolved in it. Like NaOH is a deliquescent substance.

Solvation: It is the phenomenon where a substance get surrounded by solvent molecules. If solvent is water it is known as hydration.

Thus the answer is

effloresce : where a substance loses water.

i think it's letter a.