Concept:Safe drinking water must be free from harmful germs and impurities.Explanation:Chlorine kills bacteria and other microbes in water.Boiling destroys pathogens by heat.Distillation removes impurities and microbes through evaporation and condensation.Alum only helps to settle suspended solids; it does not kill germs.Therefore, adding alum alone does not make water fit for drinking.Answer:Option D – Addition of alum.