[CBSE AIPMT 1998]
Correct Option (b)
Reason : Free-living (non-symbiotic) bacteria like Azotobacter and Bacillus polymyxa fix atmospheric nitrogen and make it available to crop plants. VAM (Vasicular Arbuscular Mycorrhizae) is an endosymbiosis between fungi and roots of higher plants.
Anabaena is a cyanobacterium (blue-green algae) which live solitary or in association with other plant and can fix atmospheric N₂.
Rhizobium bacterium makes symbiotic association with leguminous plants.