The amount of oxygen consumed by the aerobic bacterias which cause the aerobic biological decomposition of sewage, is known

B.O.D. (Bio-Chemical Oxygen Demand) refers to the amount of oxygen required by aerobic bacteria to decompose organic matter present in sewage.

During this process, microorganisms consume oxygen while breaking down organic pollutants biologically.

Hence, it directly measures the oxygen consumed by aerobic biological activity in wastewater.

D.O. (Dissolved Oxygen) represents the oxygen available in water, not the oxygen consumed.

C.O.D. (Chemical Oxygen Demand) measures oxygen required for chemical oxidation of organic matter, not specifically biological decomposition.

Therefore, the correct term describing oxygen used by aerobic bacteria is B.O.D.