The correct option is
B Eubacteria
A. Eubacteria
Solution : Nitrogen fixers generally include bacteria (unicellular) (e.g., Rhizobium) which will only belong to the eubacteria from the above options and cannot be included in algae or plantae (eukaryotic) or protista (eukaryotic). Eubacteria includes cyanobacteria.
So the correct answer is "Eubacteria".