Which of the following is a free living soil nitrogen fixing bacteria?

Azospirillum

The correct answer is Option (3) → Azospirillum

Biofertilisers are organisms that enrich the nutrient quality of the soil. The main sources of biofertilisers are bacteria, fungi and cyanobacteria. The nodules on the roots of leguminous plants formed by the symbiotic association of Rhizobium. These bacteria fix atmospheric nitrogen into organic forms, which is used by the plant as nutrient. Other bacteria can fix atmospheric nitrogen while free-living in the soil (examples Azospirillum and Azotobacter), thus enriching the nitrogen content of the soil.

Fungi are also known to form symbiotic associations with plants(mycorrhiza). Many members of the genus Glomus form mycorrhiza.

Cyanobacteria, also known as blue-green algae, are a group of photosynthetic bacteria that can fix atmospheric nitrogen into a form that plants can utilize. They form symbiotic associations with certain plants, such as rice and some aquatic plants, where they provide a source of fixed nitrogen. e.g. Anabaena, Nostoc, Oscillatoria, etc.