The correct answer is $\boxed{\text{B. 225 ppm}}$.
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 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 of sewage can be determined by incubating a known volume of the sample at a constant temperature for a specified period of time, and then measuring the amount of oxygen that is consumed. The BOD is expressed in milligrams of oxygen per liter (mg/L).
In the question, a 2% solution of sewage is incubated for 5 days at 20°C. The depletion of oxygen was found to be 5 ppm. This means that 5 milligrams of oxygen were consumed per liter of sewage per day.
To calculate the BOD of the sewage, we multiply the depletion of oxygen by the number of days that the sample was incubated, and then divide by the volume of the sample. In this case, the BOD is:
$$BOD = 5 \text{ mg/L} \times 5 \text{ days} \div 0.02 \text{ L} = 225 \text{ mg/L}$$
Therefore, the correct answer is $\boxed{\text{B. 225 ppm}}$.
The other options are incorrect because they do not represent the correct units of measurement for BOD.