Biochemical oxygen demand refers to -

The correct answer is Option (4) → Rate of uptake of oxygen by micro-organisms in water

Biochemical oxygen demand (BOD) is the amount of oxygen that micro-organisms would use up if they broke down all the organic matter in one litre of water. A high BOD means much organic matter is present, so the water is more polluted.

(1) Rate of consumption of carbon dioxide by micro-organisms in water — Incorrect. BOD is about oxygen, not carbon dioxide.

(2) Rate of uptake of oxygen by plants in land — Incorrect. BOD is measured in a water sample, and the oxygen is used by micro-organisms, not by land plants.

(3) Rate of release of oxygen by micro-organisms in water — Incorrect. The oxygen is taken up, not released. This is the opposite of what BOD means.

(4) Rate of uptake of oxygen by micro-organisms in water — Correct. This is how the BOD test measures it.

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. The greater the BOD of waste water, more is its polluting potential.”