The correct answer is A. 50 ppm.
Biological 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 an important indicator of water quality, as high BOD levels can indicate that the water is polluted with organic matter.
The BOD of a sample of sewage can be determined by incubating the sample at a constant temperature for a specified period of time and measuring the amount of oxygen that is consumed. The BOD5 is the amount of oxygen consumed in five days at 20°C.
In this case, the depletion of oxygen is found to be 5 ppm after incubating a 2.5% solution of sewage sample for 5 days at 21°C. This means that the BOD5 of the sewage is 50 ppm.
The other options are incorrect because they do not take into account the fact that the sample is a 2.5% solution of sewage. If the sample were a 1% solution, the BOD5 would be 25 ppm. If the sample were a 5% solution, the BOD5 would be 100 ppm.