A free-living anaerobic bacterium capable of fixing nitrogen is _______

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.