ANS:D - 5 day

The standard Biochemical Oxygen Demand (B.O.D.) test for water is typically conducted for 5 days. In the B.O.D. test, water samples are collected and incubated in a closed container at a specified temperature (usually 20°C) for a period of 5 days. During this time, aerobic microorganisms present in the water sample metabolize the organic matter, consuming dissolved oxygen in the process. The decrease in dissolved oxygen levels over the 5-day incubation period is measured, and the B.O.D. is expressed in milligrams of oxygen per liter of water (mg/L). The 5-day B.O.D. test period is standardized and widely accepted as it provides a reliable measure of the organic pollution load in water bodies and the effectiveness of wastewater treatment processes over a reasonable time frame. It allows for the assessment of the oxygen demand of organic matter and helps evaluate the impact of pollution on aquatic ecosystems.