Which organism(s) play an important role in the nitrogen cycle?

The correct options are
A Rhizobium
B Nostoc
C Anabena
$\bullet$ Rhizobium is found in the root nodules of leguminous plants. These bacterias convert the atmospheric nitrogen into nitrates which can be directly absorbed by the plants.
$\bullet$ Anabena is a blue-green algae that also helps in the nitrogen fixation process.
$\bullet$ Nostoc is a cyanobacteria that also helps in nitrogen fixation.
$\bullet$ Plasmodium is parasite which causes a disease called malaria.