Biological Oxygen Demand (BOD) is a standard criterion for

The answer is C. BOD is the amount of dissolved oxygen that aerobic microorganisms consume while breaking down organic matter in a water sample, usually measured over 5 days at 20 degrees Celsius (BOD5). A high BOD means a heavy organic load and poor water quality, so BOD is a core indicator of pollution in rivers, lakes and sewage.

Why the others fail:

• A is wrong because blood oxygen is assessed through partial pressure of oxygen and oximetry, not through any microbial oxygen-demand test.
• B is wrong because forest oxygen dynamics are studied through biomass, photosynthesis and carbon-cycle measurements, not BOD.
• D is wrong because high-altitude hypoxia relates to atmospheric pressure and physiology, and BOD has no role there.

Concept: water quality monitoring under the Water (Prevention and Control of Pollution) Act, 1974, with CPCB tracking BOD in rivers such as the Ganga. For Mains, link it to sewage treatment, eutrophication and river-cleaning policy.

Trap: candidates mix BOD with COD (chemical oxygen demand). COD uses a strong chemical oxidant, runs in hours, and captures non-biodegradable matter too, so it is not a biological measure.