The correct answer is $\boxed{\text{B}}$.
The biochemical oxygen demand (BOD) is a measure of the amount of oxygen that is used by microorganisms to break down organic matter in water. It is a useful indicator of the quality of water, as high BOD levels can indicate that the water is polluted with organic matter.
The BOD of a sample is determined by incubating it for a set period of time at a specified temperature and measuring the amount of dissolved oxygen that is consumed. The higher the BOD, the more organic matter is present in the water.
In this case, a 2% solution of sewage sample is incubated for 5 days at 20°C and the dissolved oxygen depletion was found to be 8 mg/l. This means that 8 mg of oxygen was consumed per liter of water over the course of 5 days. To calculate the BOD, we multiply the dissolved oxygen depletion by the dilution factor and the incubation time. The dilution factor is 1/0.02 = 50, and the incubation time is 5 days. Therefore, the BOD of the sewage is 8 mg/l * 50 * 5 = 200 mg/l.
The other options are incorrect because they do not take into account the dilution factor or the incubation time.