Why can bleaching powder lose its effectiveness when kept open in air?

Correct answer: It can be affected by moisture and carbon dioxide

Bleaching powder is a reactive salt whose available chlorine can decrease during prolonged exposure to air. Moisture and carbon dioxide promote decomposition or conversion of active constituents into less effective substances. Consequently, an open sample gradually loses bleaching and disinfecting power. Option A is correct. The alternatives involving gold, pure water or iron have no chemical basis and do not explain loss of activity.