Which of the following groups of plants are highly useful in increasing soil fertility?

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?

Enzyme nitrogenase is responsible for

The first stable product of fixation of atmospheric nitrogen in leguminous plants is

Name the most crucial enzyme found in root nodules for $N_2$ fixation. Does it require a special pink-coloured pigment for its functioning? Elaborate.

Which of the following reactions shows nitrogen fixation?