In nitrogen cycle Nitrosomonas & Nitrobacter bacteria are responsible for nitrification) During nitrification, the nitrogenous wastes from dead plants and animals are converted into ammonia by the action of bacteria such as Bacillus ramosus, Clostridium spp. etc.
The microbes which carry out biological nitrogen fixation are commonly called biological nitrogen fixers. for eg: Rhizobium and Azotobacter.
So, the correct answer is 'Nitrosomonas & Nitrobacter'.