Explanation

(c)
$CaSO_{4}.\ 2H_{2}O\overset{\ \ \ \ \ \ 120{^\circ}\ \ \ \ \ \ }{\rightarrow}CaSO_{4}.\frac{1}{2}H_{2}O\overset{\ \ \ \ \ \ 200{^\circ}\ \ \ \ \ \ }{\rightarrow}CaSO_{4}$
gypsum plaster of Paris anhydrite or dead burnt plaster
The anhydrous $CaSO_{4}$ is called dead burnt plaster because it does not set like plaster of Paris when moistened with water.