BOD stands for:

The correct answer is Option (2) → Biochemical oxygen demand

(1) Biological oxygen demand — Incorrect. The first word is not “biological”. NCERT expands BOD as biochemical oxygen demand.

(2) Biochemical oxygen demand — Correct. BOD is the amount of oxygen that bacteria would use if they oxidised all the organic matter in one litre of water. It measures indirectly how much organic matter the water holds. The more BOD the waste water has, the more polluting it is. In sewage treatment the microbes in the aeration tank eat the organic matter, so the BOD falls.

(3) Biodiversity and oxygen demand — Incorrect. Biodiversity has nothing to do with this test. This expansion does not exist.

(4) Biological oxygen damage — Incorrect. BOD is a demand for oxygen. It is not a damage. This expansion does not exist.

NCERT Extract:
Microbes in Sewage Treatment – “… While growing, these microbes consume the major part of the organic matter in the effluent. This significantly reduces the BOD (biochemical oxygen demand) of the effluent. 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.”