Hard water is not available for an experiment. Some salts are given below: (I) Sodium chloride (II) Sodium sulphate (III) Calcium chloride (IV) Calcium sulphate (V) Potassium chloride (VI) Magnesium sulphate. Select from the following a group of these salts, each member of which may be dissolved in water to make it hard.

Hardness of water is caused by the presence of dissolved calcium ($Ca^{2+}$) and magnesium ($Mg^{2+}$) salts, such as chlorides, sulphates, and hydrogencarbonates. Among the given salts, calcium chloride (III), calcium sulphate (IV), and magnesium sulphate (VI) contain calcium and magnesium ions. Dissolving these in water will make it hard. Sodium and potassium salts (I, II, V) do not cause hardness in water. Therefore, group III, IV, and VI is the correct choice.