The correct expansion of BOD is:

The correct answer is Option (3) → Biochemical Oxygen Demand

BOD is the amount of oxygen that would be used if all the organic matter in one litre of water were oxidised by bacteria. It is a measure of the organic matter present in the water. The greater the BOD of waste water, the more polluting it is. In sewage treatment the microbes lower the BOD.

(1) Biogeochemical Oxygen Demand — Incorrect. There is no such term. Biogeochemical cycles are a different topic.

(2) Biological Oxygen Demand — Incorrect. This is a loose form of the name. The standard term is Biochemical Oxygen Demand.

(3) Biochemical Oxygen Demand — Correct. This is the full form of BOD.

(4) Biodegradable Oxygen Demand — Incorrect. There is no such term.

NCERT Extract:
Microbes in Sewage Treatment – …