Biochemical Oxygen Demand (B.O.D.) of safe drinking water must be

Biochemical Oxygen Demand (B.O.D.): Biochemical Oxygen Demand is a measure of the amount of oxygen that microorganisms will consume while decomposing organic matter in water. It is an important parameter for assessing water quality.

Safe Drinking Water: For water to be considered safe for drinking, it should have very low levels of organic matter and microbial contamination. Therefore, the B.O.D. of safe drinking water should be as close to zero as possible.

Explanation of Options:

Option A: nil - Indicates that there is no measurable oxygen demand, meaning the water is free from organic contaminants. This is the ideal and safest level for drinking water.

Option B: 5 - A B.O.D. level of 5 mg/L indicates some presence of organic matter, which is higher than acceptable for safe drinking water.

Option C: 10 - A B.O.D. level of 10 mg/L is too high for safe drinking water and indicates significant organic pollution.

Option D: 15 - A B.O.D. level of 15 mg/L is far too high and indicates severe contamination by organic matter.

Therefore, the Biochemical Oxygen Demand (B.O.D.) of safe drinking water must be nil.