The correct option is D heating gypsum to 120∘C.
The chemical name of plaster of Paris is calcium sulphate hemihydrate(CaSO4.12H2O). It is obtained by heating gypsum at 120∘ C. The reaction involved is:
CaSO4.2H2O(s)120∘C−−−→CaSO4.12H2O(s)+32H2O(l) Gypsum Plaster of paris