Nitrifying bacteria convert
Nitrification is the phenomena of conversion of ammonium nitrogen to nitrate nitrogen. It is performed in two steps- nitrite formation and nitrate formation. In the first step, ammonium ions are oxidized to nitrites by bacteria Nitrosomonas. In the second step, nitrites are changed to nitrates by Nitrobacter. So this cycle is carried out by these nitrifying bacterias. Thus, the correct answer is option D.