Plaster of Paris in contact with water sets to a hard mass whose composition is: 

The name of plaster of paris is (POP). Calcium sulphate, or plaster of Paris, is not water soluble. Therefore, when plaster of Paris is exposed to water, it solidifies into a solid mass. Here, we'll analyse the hard mass that forms when plaster of Paris comes into touch with water.

Plaster of Paris is a chemical whose name is Calcium sulphate hemihydrate and when it is added to water, the reaction takes place as-

${\mathrm{CaSO}}_4.\frac{1}{2}{\mathrm{H}}_2\mathrm{O}+\frac{3}{2}{\mathrm{H}}_2\mathrm{O}\to {\mathrm{CaSO}}_4.2{\mathrm{H}}_2\mathrm{O}$

From the above chemical reaction, we can see that when the plaster of Paris reacts with water it gives ‘gypsum’ whose chemical composition is ${\mathrm{CaSO}}_4.2{\mathrm{H}}_2\mathrm{O}$.

Hence option (C) is correct.