Which of the following is/ are used as disinfectant of water?

All three are recognised water disinfectants, so the answer is D. Disinfection means killing or inactivating pathogens such as bacteria, viruses and protozoa, and each agent does it differently.

• Ozone (A): a strong oxidant generated on site by passing oxygen through an electric discharge. It is effective against Cryptosporidium, which chlorine handles poorly, but it leaves no residual, so a secondary disinfectant is needed in the distribution network.
• Chlorine dioxide (B): used where taste, odour or trihalomethane formation is a concern. It acts well across a range of pH values.
• Chloramine (C): formed by reacting chlorine with ammonia. It is a weaker but longer-lasting disinfectant, which is why it maintains residual disinfection in long pipelines.

Mains connection: water-borne diseases and urban water supply under GS-II and GS-III, including the Jal Jeevan Mission.

Trap: students pick A or B because they sound familiar and then lose the mark on D. Each option is individually correct, so D is right.