The correct option is B Anabaena
Anabeana and Rhizobium fix atmospheric nitrogen as a symbiotic association with plants. Azolla is a pteridophytic fern which symbiotically asscociates with a cyanobacterium called anabaena and leguminous plants associate with Rhizobium which fixes atmospheric nitrogen. Nostoc and Azospirillium are a free living cyanobacterium which also sequester atmospheric nitrogen.