The correct answer is D. Chloramines.
Chloramines are a group of chemical compounds that are formed when chlorine reacts with ammonia. They are used as disinfectants in water treatment because they are more stable than chlorine and do not react as readily with organic matter. Chloramines are also less likely to cause taste and odor problems than chlorine.
Carbon dioxide, bleaching powder, and sulfur dioxide are not effective de-chlorination agents. Carbon dioxide is a gas that is not soluble in water. Bleaching powder is a mixture of calcium hypochlorite and calcium chloride. Calcium hypochlorite is a strong oxidizing agent that can react with organic matter to form harmful compounds. Sulfur dioxide is a gas that is soluble in water. It can react with chlorine to form chloramines, but it is not a very effective de-chlorination agent.
In conclusion, the correct answer is D. Chloramines.