Which of the following is a free living aerobic non - photosynthetic nitrogen-fixer ?
Correct option is B. $$Azotobacter$$
Azotobacter is a free-living nitrogen-fixing bacterium, which is used as a biofertilizer in the cultivation of most crops. Azotobacter respires aerobically, receiving energy from redox reactions, using organic compounds as electron donors. Azotobacter can use a variety of carbohydrates, alcohols, and salts of organic acids as sources of carbon.
Rhizobium fixes nitrogen by symbiotic association. Nostoc is photosynthetic. Azospirillum is anaerobic.
So, the correct option is 'Azotobacter'.