The setting of Plaster of Paris involves: 

The production of calcium hydrate is what causes the plaster of Paris to solidify. The opposite of the gypsum's dehydration process is the hydration reaction, which occurs when the plaster of Paris is hydrated. Plaster of Paris hardens quickly after being prepared into a thin paste with water.
Due to the fact that this process is exothermic, or that heat is emitted, a small expansion occurs during it.
The following is the reaction:

$2{\mathrm{CaSO}}_4.{\mathrm{H}}_2\mathrm{O}\stackrel{\mathrm{setting}}{\to }{\mathrm{CaSO}}_4.2{\mathrm{H}}_2\mathrm{O}$

Plaster of Paris sets by hydration as a result of the creation of a solid, crystalline hydrate. - The opposite of gypsum's dehydration, the hardening of plaster of Paris is a hydration response. When produced into a thin paste with water, plaster of paris hardens in a short period of time.

Therefore the correct answer is option (D).