The correct option is B Rhizobium
Legumes have a symbiotic relationship with bacteria called Rhizobia (Rhizobium). Soya bean is a legume.
Rhizobium is a nitrogen-fixing bacteria for leguminous crops.
These microorganisms fix the atmospheric nitrogen in the roots (root nodules) and make it available to plants, which plants cannot fix directly and makes the soil fertile and also increases the plant health.
Nostoc, Azotobactor, and Azospirillum are free-living nitrogen-fixing bacteria.