Which among the following is a nitrogen-fixing organism?

The correct answer is Option (4) → All of these

Cyanobacteria (blue-green algae) are autotrophic microbes found in water and on land. Many of them can fix atmospheric nitrogen, and in paddy fields they serve as an important biofertiliser.

Option (1) Oscillatoria — Correct statement, but it is only one of the correct points, so on its own it is not the answer. Oscillatoria is a cyanobacterium named by NCERT among those that fix atmospheric nitrogen.

Option (2) Anabaena — Correct statement, but it is only one of the correct points, so on its own it is not the answer. Anabaena is a nitrogen-fixing cyanobacterium.

Option (3) Nostoc — Correct statement, but it is only one of the correct points, so on its own it is not the answer. Nostoc is also a nitrogen-fixing cyanobacterium.

All three are nitrogen-fixing organisms, so Option (4) is the answer.

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.”