Which one is a non-symbiotic nitrogen fixer?

$A-$ The microbe $Frankia$ produces nitrogen-fixing nodules on the roots of non-leguminous plants.
$R-$ Both $Rhizobium$ and $Frankia$ are free-living in soil,but as symbionts,can fix atmospheric nitrogen.

Which of the following is a free-living aerobic bacterium capable of nitrogen fixation?

The function of leghaemoglobin during $N_2$-fixation is

During biological nitrogen fixation,which substance protects the enzyme nitrogenase from the harmful effects of oxygen?

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