Which of the following enzymes is required for nitrogen fixation?

Nitrogenase is the key enzyme complex responsible for biological nitrogen fixation - it converts atmospheric N2 into ammonia (NH3). This enzyme is found in nitrogen-fixing bacteria like Rhizobium. Nitrogen deaminase and Nitrodioxidase are not real enzymes involved in this process, and amino acid decarboxylase is involved in amino acid metabolism, not nitrogen fixation.