The correct answer is D. Bleaching powder is prepared by passing chlorine over quicklime.
Quicklime is calcium oxide, which is produced by heating limestone (calcium carbonate) to a high temperature. When chlorine is passed over quicklime, it reacts to form calcium hypochlorite, which is the active ingredient in bleaching powder.
The other options are incorrect because they do not produce calcium hypochlorite. Option A, passing oxygen over slaked lime, would produce calcium oxide and water. Option B, passing carbon dioxide over slaked lime, would produce calcium carbonate. Option C, passing carbon dioxide over quicklime, would produce calcium oxide and carbon monoxide.
Bleaching powder is a white, odorless powder that is used as a bleaching agent, disinfectant, and deodorizer. It is also used in the production of paper, textiles, and other products.