Gypsum on heating to about \[{\rm{120^\circ C}}\] forms a compound that has the chemical composition represented by
A. \[{\rm{2CaS}}{{\rm{O}}_{\rm{4}}}{\rm{.}}{{\rm{H}}_{\rm{2}}}{\rm{O}}\]
B. \[{\rm{CaS}}{{\rm{O}}_{\rm{4}}}\]
C. \[{\rm{CaS}}{{\rm{O}}_{\rm{4}}}{\rm{.}}{{\rm{H}}_{\rm{2}}}{\rm{O}}\]
D. \[{\rm{2CaS}}{{\rm{O}}_{\rm{4}}}{\rm{.3}}{{\rm{H}}_{\rm{2}}}{\rm{O}}\]

Hint: The chemical formula of gypsum is \[{\rm{CaS}}{{\rm{O}}_{\rm{4}}}{\rm{.2}}{{\rm{H}}_{\rm{2}}}{\rm{O}}\]. It is a soft sulphate mineral compound of calcium sulphate dihydrate. Gypsum when heated at 393 K gives rise to calcium sulphate.

Complete Step by Step Solution:
Gypsum is widely mined and is utilised as a fertiliser and as the major component in numerous forms of plaster, blackboard or sidewalk chalk, and drywall. It is a compound of calcium sulphate dihydrate. This compound, when heated at 393 K, loses crystallisation water and forms calcium sulphate's calcium hemihydrate. Hemihydrate of calcium sulphate is also called plaster of Paris.

The reaction occurs as follows:
 \[2(CaS{O_4}.2{H_2}O)\]\overset{393K}{\rightarrow}\[2(CaS{O_4}).{H_2}O\]\[ + 3{H_2}O\]
The temperature must not rise above 473K which will lead to a loss of whole water.
Anhydrous salt will be left called dead burnt plaster having no plaster of Paris properties.

Plaster of Paris is a white powder. On mixing it with water, it forms a plastic mass which sets into a hard solid in 5-15 minutes. This process is called the setting of plaster of Paris. During the setting, there is a slight increase in volume which helps it take the shape of any mould to which it is added.
So, option A is correct.

Note: Plaster of Paris is used for producing moulds for pottery, ceramics, etc. It is used for making statues, models, and other decorative materials. It is used in surgical bandages known as plasters for setting broken and fractured bones in the body because it immobilises the affected part of the body having a fracture or sprain.