Correct answer is (a) Excess sludge generation
For explanation I would say: Heterotrophic bacteria nitrifies the oxidation of inorganic and organic reduced forms of Nitrogen to nitrate which is known as heterotrophic nitrification, excess sludge generation creates a problem and is a major disadvantage of heterotrophic nitrification, whereas, less sludge generation would not create a problem for heterotrophic nitrification, oxygen is required for oxidation process, so, that wouldn’t create a problem, suitable carbon source is required for heterotrophic nitrification.