Which one among the following biofertilizers does not fix atmospheric nitrogen ?

The correct option is Glomus.

To understand why this is the correct answer, let's look at the functions of each of the given organisms in relation to biofertilization and nitrogen fixation:

1. Rhizobium: This is a symbiotic bacterium that lives inside the root nodules of leguminous plants. It actively fixes atmospheric nitrogen ($N_2$) into ammonia, which the plant can readily absorb.

2. Azospirillum: This is a free-living or associative nitrogen-fixing bacterium found in the rhizosphere of various grasses and cereal crops. It converts atmospheric nitrogen into a form usable by plants.

3. Oscillatoria: This is a genus of filamentous cyanobacteria (blue-green algae). Many cyanobacteria, including species of Oscillatoria, are capable of fixing atmospheric nitrogen, either in free-living states or in symbiotic associations.

4. Glomus: This is a genus of arbuscular mycorrhizal (AM) fungi. Unlike the other three organisms, Glomus does not possess the nitrogenase enzyme complex required to fix atmospheric nitrogen. Instead, it forms a symbiotic association with plant roots to primarily facilitate the absorption of phosphorus, water, and other essential nutrients from the soil. Therefore, it is a biofertilizer but not a nitrogen-fixing one.