Which of the following is a free-living,aerobic,and non-photosynthetic nitrogen-fixing bacterium?

We find that $Rhizobium$ forms nodules on the roots of leguminous plants. Also,$Frankia$,another microbe,forms nitrogen-fixing nodules on the roots of the non-leguminous plant $Alnus$. Can we artificially induce the property of nitrogen fixation in a plant,whether leguminous or non-leguminous?

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

$N_2$ fixation is

The biological process carried out by $Rhizobium$ is called:

Nitrogen fixing genes are called