Autotrophic microbe for fixing atmospheric nitrogen:

Oscillatoria

The correct answer is Option (3) -Oscillatoria

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. Currently, in our country, a number of biofertilisers are available commercially in the market and farmers use these regularly in their fields to replenish soil nutrients and to reduce dependence on chemical fertilisers.

Trichoderma and Aspergillus are fungi .

Rhizobium is heterotrophic bacteria, as symbiont, can fix atmospheric nitrogen.