The formula of Plaster of Paris is

Correct answer: (b) (CaSO4)2.H2O

Explanation

Plaster of Paris is calcium sulphate hemihydrate, CaSO₄·½H₂O: one molecule of water for every two of calcium sulphate.

Written in whole numbers, that is (CaSO₄)₂·H₂O. CaSO₄·2H₂O is gypsum, from which it is made.