Concept:The part labeled I is a root nodule, which contains nitrogen-fixing bacteria.
Explanation:Root nodules are swellings found on the roots of leguminous plants.
These nodules provide a home for bacteria that can fix atmospheric nitrogen.
The specific bacterium present in root nodules is Rhizobium.
It lives symbiotically with the host plant.
Nitrosomonas and Nitrobacter are nitrifying bacteria found in soil, not in root nodules.
Azotobacter is a free-living nitrogen-fixing bacterium, not associated with root nodules.
Answer:D. Rhizobium
