The correct option is B Nitrobacter
The conversion of ammonia to nitrate is performed primarily by soil living bacteria and other nitrifying bacteria. In the primary stage of nitrification, the oxidation of ammonium (NH4+) is performed by bacteria such as the nitrosomonas species, which converts ammonia to nitrites (NO2-). Other bacterial species, such as nitrobacter, are responsible for the oxidation of the nitrites into nitrates (NO3-). It is important for the ammonia to be converted to nitrates, because accumulated nitrites are toxic to plant life.