What is the composition of the enzyme nitrogenase?

Which of the following is an example of a free-living and anaerobic $N_2$-fixing bacterium?

Choose the incorrect pair$(s)$:
$(I)$ Free-living nitrogen-fixing aerobic microbes - Beijerinckia
$(II)$ Rhodospirillum - Aerobic microbes
$(III)$ Bacillus - Free-living
$(IV)$ Cyanobacteria - Anabaena and Azotobacter

The chemical fertilizer required for better rhizobial nitrogen fixation is

Which bacteria fix atmospheric nitrogen while they are free-living in the soil?

Free-living bacteria that perform nitrogen fixation are found in .........