Concept:Nitrification is the biological oxidation of ammonia to nitrate by specialized bacteria.Explanation:The nitrogen cycle has two nitrification steps.First, ammonia (NH3) is oxidized to nitrite (NO2−) by Nitrosomonas.Second, nitrite is oxidized to nitrate (NO3−) by Nitrobacter.Therefore, Nitrobacter directly carries out the oxidation of nitrite to nitrate.Azotobacter and Rhizobium are nitrogen-fixing bacteria; they convert atmospheric nitrogen into ammonia, not nitrate.Answer:A. Nitrobacter