Which one of the following elements plays an important role in biological nitrogen fixation?

$A-$ 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,and non-photosynthetic nitrogen-fixing bacterium?

$Azotobacter$ and $Bacillus \text{ } polymyxa$ are:

How many electrons and protons are required to fix a molecule of dinitrogen $(N_2)$ (in each)?

What is nitrogen fixation?