Alert - the correct answer should be 2.

The disinfecting action of chlorine in water-purification is mainly due to the formation of hypochlorous acid when chlorine is added to water, not the formation of nascent oxygen.

Option 1 and 4 are incorrect as the addition of chlorine to water does not produce hydrochloric acid or hydrogen, respectively. Although hydrochloric acid is a powerful acid, it does not have disinfecting properties; hydrogen is a gas and doesn`t have disinfecting properties either.

Option 3, the formation of nascent oxygen, is the provided incorrect answer. Oxygen doesn`t play much role in chlorination; it is not the main chemical responsible for disinfection here.

Option 2, the formation of hypochlorous acid, is the correct answer. When chlorine dissolves in water, it reacts with water to form hypochlorous acid and hydrochloric acid. Hypochlorous acid, not hydrochloric acid, is the active disinfecting agent in this reaction. It`s known to inactivate a wide range of harmful microorganisms in water making it safe for consumption.

Thus, the ability of chlorine to disinfect water is mostly linked to hypochlorous acid.