The correct option is A nitrification
In nitrification, the ammonia is converted into nitrite with the help of Nitrosomonas bacteria. Later, the produced nitrites are converted into nitrates by Nitrobacter. Denitrification is the process of converting the nitrate back into gaseous nitrogen by bacteria such as Pseudomonas denitrificans and Thiobacillus denitrificans.
During the process of nitrogen fixation, atmospheric nitrogen is converted into ammonia or other usable nitrogenous compounds like nitrates.