Concept:Nitrosomonas bacteria help convert ammonia into nitrites, a key step in nitrification.Explanation:In the nitrogen cycle, Nitrosomonas oxidises ammonia (NH3) into nitrites (NO2−).This conversion is a part of the nitrification process.Nitrification is the biological oxidation of ammonia to nitrite and then to nitrate.Thus, Nitrosomonas does not carry out ammonification, denitrification, or nitrogen fixation.Answer:B. Nitrification