Plaster of Paris is hardened by

1. liberating CO2 ii.hydration iii, dehydration iv. changing into CaCO3

Plaster of Paris(CaSO4)₂.H₂O is an important compound which is prepared by heating gypsum(CaSO4)₂.2H₂O.In order to hardened it and solidifying it,it is mixes with water.This mixing of water with plaster causes an exothermic chemical reaction that releases heat. This heat helps to harden the Plaster of Paris allowing it to set.Thus it is hardened by hydration.So the option (ii) hydration will be the correct answer.