Right option is (c) Biochemical oxygen demand
For explanation I would say: The amount of oxygen required by bacteria to break down the organic matter present in a certain volume of a sample of water is called as Biochemical Oxygen Demand (BOD). The amount of BOD in water is a measure of the amount of organic material in the water, in terms of how much oxygen will be required to break it down biologically. Clean water has BOD value less than 5 ppm whereas highly polluted water has BOD value of 17 ppm or more.