Correct answer is (b) Nitrogen
Explanation: During algal biomass harvesting, there are chances of nitrogen contamination is the main source of eutrophication of surface water, during microbiological examination, the relative concentration of organism at the time of stress is useful in determining the changes in effluent quality of water, salty water is found abundantly in seas, pure water doesn’t contain any impurities, however these does not cause eutrophication.