What does a higher BOD value of a water sample indicate regarding its…

The correct answer is Option (1) → High pollution

BOD (biochemical oxygen demand) is the amount of oxygen that would be used up if all the organic matter in one litre of water were oxidised by bacteria. More organic matter means more oxygen is needed, so BOD is an indirect measure of the organic matter in water.

Option (1) High pollution — Correct. A higher BOD means more organic matter in the water, so the water is more polluted. The book says the greater the BOD of waste water, the more is its polluting potential.

Option (2) No pollution — Incorrect. Water with no pollution has very little organic matter, so its BOD is low, not high.

Option (3) Moderate pollution — Incorrect. A higher BOD points to a higher level of pollution. It does not show a moderate level.

Option (4) Pollution level cannot be determined — Incorrect. BOD is the very test used to judge pollution by organic matter. Its value does tell the pollution level.

NCERT Extract:
Microbes in Sewage Treatment – “… 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. The sewage water is treated till the BOD is reduced. The BOD test measures the rate of uptake of oxygen by micro-organisms in a sample of water and thus, indirectly, BOD is a measure of the organic matter present in the water. The greater the BOD of waste water, more is its polluting potential.”