Concept: Nitrogen-fixing microorganisms are capable of transforming atmospheric nitrogen into fixed nitrogen.
Two kinds of nitrogen-fixing bacteria are recognized.
The first kind is the free-living (non-symbiotic) bacteria including cyanobacteria and some genera such as Azotobacter, Clostridium, etc.
Clostridium includes the genus of anaerobic, gram-positive bacteria.
Whereas Azotobacter refers to the genus of aerobic gram-negative nitrogen-fixing bacteria.
Hence, option 3 is correct.