In the neighbourhood of your school, hard water required for an experiment is not available. Select from the following groups of salts available in your school, a group each member of which, if dissolved in distilled water, will make it hard:

Hardness of water is caused by the presence of dissolved hydrogen carbonates, chlorides, and sulphates of calcium ($Ca^{2+}$) and magnesium ($Mg^{2+}$) ions. When calcium chloride ($CaCl_2$) and magnesium chloride ($MgCl_2$) are dissolved in distilled water, they dissociate to release $Ca^{2+}$ and $Mg^{2+}$ ions, which react with soap to form an insoluble precipitate (scum), thereby making the water hard. Sodium ($Na^+$) and potassium ($K^+$) salts (like $NaCl$ and $KCl$) do not cause hardness in water. Thus, option (d) is the correct choice, while options (a), (b), and (c) are incorrect because they contain sodium or potassium salts.