Explanation
- A
Measuring oxygen levels in blood
Measuring oxygen in blood is a medical/physiological test (pulse oximetry, arterial blood gas), not BOD. Biological oxygen demand is not a haematology criterion. This option is not the key.
- B
Computing oxygen levels in forest ecosystems
Forest ecosystems are not assayed by BOD. Terrestrial oxygen and productivity use other ecological measures. This option is therefore wrong.
- C
Pollution assay in aquatic ecosystems
Biological Oxygen Demand is the amount of dissolved oxygen consumed by microbes while decomposing organic matter in water over a fixed period (commonly five days at 20°C). High BOD means high organic pollution in rivers, lakes and effluents. It is the standard pollution assay for aquatic ecosystems, so (c) is the official key.
- D
Assessing oxygen levels in high altitude regions
High-altitude oxygen is a physiological and meteorological issue, not BOD. This option is not the key.
Summary. Official key is (c) Pollution assay in aquatic ecosystems. BOD tracks how much oxygen decomposers take from a water sample, so it grades organic pollution of sewage and rivers. It is not a blood test, a forest-oxygen index, or an altitude measure. COD is a related but chemical oxidant test. Only (c) names BOD’s use.
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UPSC has asked this before
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