Why can bleaching powder slowly lose its effectiveness when kept exposed?

Correct answer: It can react with moisture and carbon dioxide in air

Bleaching powder contains reactive chlorine compounds that provide its bleaching and disinfecting action. On prolonged exposure to humid air and carbon dioxide, these active constituents may undergo unwanted reactions and gradually lose available chlorine. Therefore, the powder becomes less effective with time. Option A is correct; the other options describe impossible or scientifically unrelated changes. It should be stored in a tightly closed, dry container.