Which of the following is $NOT$ a diazotroph?

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

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

Nitrogen fixation means

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 bacteria are free-living nitrogen fixers?
$(i)$ $Pseudomonas$ $(ii)$ $Azospirillum$
$(iii)$ $Azotobacter$ $(iv)$ $Nostoc$