Which of the following is the pair of biofertilisers?

The correct option is A $Azolla$ and BGA
$Azolla$ and BGA
$Anabaena$ which is a blue green algae (BGA) forms a symbiotic association with the leaves of $Azolla$ which is a fern. The fern provides shelter to the cyanobacteria while the latter helps in nitrogen fixation and supplies it to the fern.
$Azolla$ can coexist with certain plants. The death of the plant having Azolla causes degradation followed by the release of nitrogenous compounds to the nearby plants.

$Nostoc$ and legume
$Nostoc$ is a free-living cyanobacteria that can fix atmospheric nitrogen due to the presence of specialised cells called heterocysts.
Legumes are able to fix nitrogen only if the roots have nodules containing $Rhizobium$.
Hence, they don’t form a perfect match.

$Rhizobium$ and grasses
$Rhizobium$ is a symbiotic nitrogen fixing bacteria that colonise leguminous plants and not grasses.
Hence, they don’t form a perfect match.

$Salmonella$ and $E.coli$
$Salmonella$ and $E.coli$ are human pathogens.

Final answer
(A) $Azolla$ and BGA