Autotrophic microbe for fixing atmospheric nitrogen:

The correct answer is Option (3) → Oscillatoria

Option (1) Rhizobium — Incorrect. Rhizobium fixes nitrogen in the root nodules of legumes, but it is a heterotroph: it takes its food from the plant.

Option (2) Aspergillus — Incorrect. Aspergillus niger is a fungus used to produce citric acid. It does not fix nitrogen.

Option (3) Oscillatoria — Correct. Oscillatoria is a cyanobacterium. It makes its own food by photosynthesis and fixes atmospheric nitrogen, like Anabaena and Nostoc.

Option (4) Trichoderma — Incorrect. Trichoderma is a free-living fungus used as a biocontrol agent against plant pathogens.

NCERT Extract:
Microbes as Biofertilisers – “… Cyanobacteria are autotrophic microbes widely distributed in aquatic and terrestrial environments many of which can fix atmospheric nitrogen, e.g. Anabaena, Nostoc, Oscillatoria, etc. In paddy fields, cyanobacteria serve as an important biofertiliser. Blue green algae also add organic matter to the soil and increase its fertility.”