Option 1: "It kills germs." This is not the correct explanation for why potassium permanganate is used for purifying drinking water. While it may have some germicidal properties, its main function in water purification is not to kill germs.

Option 2: "It dissolves the impurities." This is also not the correct explanation. While potassium permanganate can help in removing certain impurities, its primary role in purifying drinking water is not to dissolve impurities.

Option 3: "It is a reducing agent." This is not the correct explanation for why potassium permanganate is used in water purification. In fact, potassium permanganate itself is an oxidizing agent, not a reducing agent.

Option 4: "It is an oxidizing agent." This is the correct explanation. Potassium permanganate is commonly used for purifying drinking water because it is an effective oxidizing agent. It reacts with various organic and inorganic compounds present in water, helping to remove impurities and pollutants. It is particularly effective in removing iron, manganese, and hydrogen sulfide, which are common water contaminants. Its strong oxidizing properties make it a useful tool in water treatment processes.