The correct option is B hard water
The hardness of water is due to the presence of calcium and magnesium salts. Soaps react with these salts to form an insoluble precipitate in water; this reduces the effectiveness of soap. Detergents do not form insoluble precipitates with the calcium and magnesium ions in hard water. They are long chains of ammonium or benzene sulphonic acids and form lather easily even with hard water. The detergents do not form an insoluble precipitate with calcium and magnesium salts in water.