Which one of the following is an aerobic non-symbiotic nitrogen-fixing bacteria?

Correct option is A

Azotobacter is a free-living, aerobic, non-symbiotic nitrogen-fixing bacterium found in neutral to alkaline soils. It fixes atmospheric nitrogen into ammonia, which helps in enriching soil fertility. In contrast, Clostridium is anaerobic, Rhizobium forms symbiotic associations with legume roots, and Beijerinckia, although aerobic, is generally less prominent than Azotobacter in agricultural contexts. Azotobacter also produces vitamins, growth-promoting substances, and contributes to biological control of plant pathogens.