What is nitrogen fixation?

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

Which of the following gene clusters in bacteria is responsible for nitrogen fixation?

Which of the following is a flowering plant with nodules containing filamentous nitrogen-fixing micro-organism?

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?

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.