The water pollution in river is measured by the dissolved amount of

Answer: (d) Oxygen

Dissolved oxygen (with BOD) measures river pollution — the dirtier the water, the lower the DO.

The idea

Fish and other water animals breathe the oxygen dissolved in water. Clean water holds plenty. When sewage enters, microbes feeding on it use that oxygen up. So the amount of dissolved oxygen left is a direct reading of how polluted a river is.

Clean river: dissolved oxygen high → Sewage enters → Microbes consume oxygen → Dissolved oxygen falls → Below about 4 mg per litre, fish are in trouble

Why the others are wrong

(a) Chlorine is added to clean water, not used to measure pollution.

Also know

Cold water holds more oxygen than warm water, so hot water discharged by power plants (thermal pollution) also lowers it. The pollution control board classes river water from A to E by use, with dissolved oxygen, BOD and coliform count as the main tests.