Which cofactor is essential for the activity of the nitrogenase enzyme?

Select the incorrect statement with respect to the biological nitrogen fixation by Rhizobium.

All of the following statements concerning the actinomycetous filamentous soil bacterium $Frankia$ are correct except that $Frankia$:

Which of the following are free-living nitrogen-fixing bacteria?

Categorize the following nitrogen-fixing microorganisms into symbiotic and free-living types:
$I-$ Rhizobium
$II-$ Azotobacter
$III-$ Beijerinckia
$IV-$ Rhodospirillum
$V-$ Frankia
$VI-$ Bacillus
$VII-$ Anabaena (free-living)
$VIII-$ Nostoc (symbiotic)
Select the correct grouping for (Symbiotic) and (Free-living) respectively.

$nif$ genes occur in