Microbes as biofertilisers

The water fern Azolla harbours the cyanobacterium Anabaena azollae in its leaf cavities.
Anabaena fixes atmospheric nitrogen, so Azolla is used as a biofertiliser in paddy fields.
Frankia forms nodules on the roots of non-leguminous plants such as Alnus.

Many members of the genus Glomus form mycorrhiza, a symbiotic association of a fungus with the roots of plants.
The fungus absorbs phosphorus from the soil and passes it to the plant.
Azotobacter is a free-living nitrogen fixer and Trichoderma is a biocontrol agent.

Biofertilisers are organisms that enrich the nutrient quality of the soil; the main sources are bacteria, fungi and cyanobacteria.
$Rhizobium$ fixes atmospheric nitrogen in the root nodules of legumes.
$Nostoc$ (like $Anabaena$ and $Oscillatoria$) is a nitrogen-fixing cyanobacterium.
Mycorrhiza is the symbiotic association of a fungus with roots, which helps the plant to absorb phosphorus.
$Agrobacterium\ tumefaciens$ is a plant pathogen that causes crown gall tumours; it is used as a vector for gene transfer in plants and does not enrich the soil.
Hence $Agrobacterium$ is not a biofertiliser.

Many members of the fungal genus $Glomus$ form mycorrhiza, a symbiotic association with the roots of higher plants.
The fungal symbiont absorbs phosphorus from the soil and passes it to the plant.
Plants with such associations also show resistance to root-borne pathogens, tolerance to salinity and drought, and an overall increase in growth.
$Anabaena$, $Rhizobium$ and $Frankia$ are nitrogen fixers; they do not help in phosphorus absorption.
Hence $Glomus$ helps in the absorption of phosphorus.

Soyabean is a leguminous plant.
The roots of legumes form nodules by symbiotic association with the bacterium $Rhizobium$.
$Rhizobium$ fixes atmospheric nitrogen into organic forms in the nodules, which the plant uses as a nutrient; so it is used as the biofertiliser for legume crops.
$Azotobacter$ and $Azospirillum$ are free-living nitrogen fixers of the soil, and $Nostoc$ is a cyanobacterium used mainly in paddy fields.
Hence the biofertiliser used for raising a soyabean crop is $Rhizobium$.

Mycorrhiza is a symbiotic association between a fungus and the roots of a higher plant.
In endomycorrhiza the fungal hyphae penetrate the cortical cells of the root (vesicular-arbuscular mycorrhiza, VAM).
Many members of the genus $Glomus$ form such mycorrhiza; the fungus absorbs phosphorus from the soil and passes it to the plant.
$Nostoc$ is a cyanobacterium, $Agaricus$ is the mushroom (a basidiomycete) and $Rhizobium$ is a nitrogen-fixing bacterium.
Hence the example of endomycorrhiza is $Glomus$.

Organic farming uses biofertilisers and natural processes in place of chemical fertilisers.
$Glomus$ is a fungus that forms mycorrhiza and helps the plant to absorb phosphorus.
$Oscillatoria$ is a nitrogen-fixing cyanobacterium that adds organic matter and nitrogen to the soil.
Earthworms break down organic matter and are used to make vermicompost.
Snails are not used; they feed on crop plants and are pests.
Hence the snail is not used in organic farming.

$Rhizobium$ is a symbiotic nitrogen fixer of the root nodules of legumes, and $Frankia$ forms nodules on the roots of non-leguminous plants such as $Alnus$; neither is associated with paddy.
$Azospirillum$ is a nitrogen-fixing bacterium that lives in loose association with the roots of grasses and cereals, including rice; it is the answer accepted in the key.
$Oscillatoria$ is a cyanobacterium; cyanobacteria ($Anabaena$, $Nostoc$, $Oscillatoria$) are also important nitrogen fixers of paddy fields.
The answer accepted in the key is $Azospirillum$.
Note: NCERT states that in paddy fields cyanobacteria serve as an important biofertiliser, so $Oscillatoria$ is given as the answer in some books; the key of the source PDF is retained here.