The correct answer is B. Sulphuric acid.
Sulphuric acid is used in the COD test because it is a strong acid that can easily dissolve organic matter. The organic matter is then oxidized by potassium dichromate, which is a strong oxidizing agent. The amount of potassium dichromate that is used is proportional to the amount of organic matter in the sewage.
The other options are incorrect because they are not strong acids. Hydrochloric acid is a weak acid, and nitric acid and citric acid are both organic acids. Organic acids are not strong enough to dissolve organic matter, so they cannot be used in the COD test.
The COD test is a measure of the amount of oxygen that is required to oxidize the organic matter in a sample. The higher the COD value, the more organic matter is present in the sample. The COD test is used to measure the pollution level of sewage and wastewater.