The correct option is Rhizobium.
Explanation
Nitrogen fixation is the biological process by which atmospheric nitrogen ($N_2$) is converted into ammonia ($NH_3$) or related nitrogenous compounds, making it accessible to plants. This process is carried out by specific prokaryotic microorganisms, which may be classified as either free-living or symbiotic.
- Rhizobium Rhizobium: Correct. Rhizobium is a symbiotic nitrogen-fixing bacterium. It establishes a mutualistic relationship specifically with leguminous plants (such as peas, beans, and lentils). The bacteria colonize the plant roots, inducing the formation of root nodules, where they convert atmospheric nitrogen into ammonia for the plant's use.
- Azotobacter Azotobacter: Incorrect. Azotobacter is a free-living, aerobic nitrogen-fixing bacterium found in the soil. It functions independently and does not form symbiotic root nodules with legumes.
- Cyanobacteria Cyanobacteria: Incorrect. Also known as blue-green algae, Cyanobacteria (e.g., Nostoc, Anabaena) can fix nitrogen. While some form symbiotic associations (e.g., Anabaena with the water fern Azolla or in cycad roots), they are not the specific symbionts found in the root nodules of leguminous crops.
- Nitrobacter Nitrobacter: Incorrect. Nitrobacter is involved in nitrification, not nitrogen fixation. It is a chemoautotrophic bacterium that oxidizes nitrite ($NO_2^-$) into nitrate ($NO_3^-$), which is a form of nitrogen readily absorbed by plants, but it does not fix atmospheric nitrogen.
Key Takeaway: Rhizobium is the distinct symbiotic bacterium responsible for fixing atmospheric nitrogen within the root nodules of leguminous plants.