Nitrogen fixation is carried out primarily by

1. Bacteria

Nitrogen fixation is carried out primarily by organism –bacteria. Further, some of these bacteria can synthesis and change N2 and form ammonia in the said procedure, now called as nitrogen fixation. These organisms –bacteria- are associated with a symbiotic relationship and free-living with either other organisms such as protozoa, termites or plants. This conversion happens in accord with the mutualistic relationship the bacteria has with its host and undergoes during this process nitrogenase is present.    

B, because u need evidence to support ur theory and claim and it needs to be correct evidence