Biochemical oxygen demand is an indicator of -

The correct answer is Option (1) → Organic matter in the water

BOD stands for biochemical oxygen demand. The more organic matter the water has, the more oxygen the microbes need. So a high BOD means the water is highly polluted with organic matter, such as sewage. In sewage treatment, the water is treated until its BOD is reduced.

(1) Organic matter in the water — Correct. BOD is an indirect measure of the organic matter present in the water.

(2) Inorganic matter in the water — Incorrect. Inorganic matter is not broken down by bacteria using oxygen, so it does not raise the BOD.

(3) Pesticides in the water — Incorrect. Pesticides like DDT are measured by their concentration. They build up along food chains (biomagnification); BOD does not measure them.

(4) Heavy metals in the water — Incorrect. Heavy metals such as mercury are toxic chemicals, not organic matter that bacteria oxidise.

NCERT Extract:
Microbes in Sewage Treatment – “… BOD refers to the amount of the oxygen that would be consumed if all the organic matter in one liter of water were oxidised by bacteria. The sewage water is treated till the BOD is reduced. The BOD test measures the rate of uptake of oxygen by micro-organisms in a sample of water and thus, indirectly, BOD is a measure of the organic matter present in the water.”