The compound which on dissolution in water makes it hard, is

Explanation

Hardness of water is due to the presence of soluble bicarbonates, chlorides and sulphates of calcium and magnesium. Bicarbonates cause temporary hardness, which can be removed by boiling the water, e.g., calcium bicarbonate [Ca(HCO₃)₂]. Chlorides and sulphates cause permanent hardness, which cannot be removed by boiling. Permanent hardness can be removed only by chemical reactions, e.g., magnesium sulphate (MgSO₄), calcium chloride (CaCl₂), etc. Therefore, option (d) is correct.