The correct option is C Nitrogen fixation
The equation represents nitrogen fixation. Free gaseous nitrogen gets reduced to ammonia. This then gets converted into nitrites and then nitrates. This way the nitrogen is made available to the plants. Nitrogenase enzyme catalyzes this reaction and this takes place in the root nodules of leguminous plants.
Reductive amination is an enzyme mediated process in which ammonium ion combines with α-ketoglutaric acid to form glutamic acid.
Denitrification is the conversion of nitrates into free nitrogen.
Nitrification is the conversion of ammonia into nitrites and then into nitrates.