If by radiation all nitrogenase enzyme is inactivated, then there will be no fixation of nitrogen in legumes. The enzyme nitrogenase is necessary for biological nitrogen fixation only. If it gets inactivated by radiation, the process of biological nitrogen fixation will get affected. Nitrogen fixation occurs through other ways also. So either phenomenon of nitrogen fixation will not stop by the absence of nitrogenase enzyme.
Neither conversion of ammonium to nitrate(nitrification) nor conversion of nitrate to nitrite require nitrogenase. Hence they will not get affected.
So, the correct answer is 'Fixation of nitrogen in legumes'.