Correct answer: Option \\\\(A\\\\).Hint: Plaster of Paris is hemihydrate of \\\\(CaSO_4\\\\) and gypsum is its dihydrateExplanation: Formula for Plaster of Paris is \\\\(CaSO_4.\\dfrac {1}{2}H_2O\\\\)Formula for Gypsum is \\\\(CaSO_4. 2H_2O\\\\)When plaster of Paris gains water, it turns into gypsum and therefore hardens, which is responsible for the setting property of plaster of Paris.The reaction of plaster of pairs with water is shown as;\\\\(\\displaystyle{\\underset{Plaster\\ of\\ Paris}{CaSO_4.\\dfrac{1}{2}H_2O} + \\underset{Water}{\\frac{3}{2}H_2O} \\longrightarrow \\underset{Gypsum}{CaSO_4.2H_2O}}\\\\)Hence, option \\\\(A\\\\), \\\\(CaSO_4.\\dfrac{1}{2}H_2O + \\dfrac{3}{2}H_2O \\rightarrow CaSO_4.2H_2O\\\\) is correct.