ANS:C - 20°C

The 5-day Biochemical Oxygen Demand (BOD₅) is typically measured at a temperature of 20°C. BOD₅ is a standard method used to assess the organic pollution level of water bodies by determining the amount of oxygen consumed by microorganisms during a 5-day incubation period at a specific temperature. While BOD can be measured at different temperatures, 20°C is the standard temperature for BOD₅ determination in most laboratory protocols and environmental regulations. At this temperature, microbial activity is sufficiently high to provide accurate measurements of organic matter degradation within a reasonable incubation period. Additionally, 20°C is considered a suitable temperature for representing typical environmental conditions in many aquatic ecosystems where BOD assessment is relevant. Therefore, BOD₅ is commonly measured at a temperature of 20°C for consistency and comparability of results across different studies and regulatory frameworks.