The symbiotic nitrogen-fixing bacterium is
Correct option is C. Rhizobium
Nitrosomonas - is a gram-negative bacteria that oxidizes Ammonia into nitrites.
Nitrobacter - is a gram-negative bacteria that oxidizes nitrite to nitrates.
Rhizobium - Is a symbiotic Nitrogen fixing bacteria in leguminous plants.
So, the correct answer is 'Rhizobium'