Rhizobium is a bacterium that fixes atmospheric nitrogen into organic…

The correct answer is Option (3) → In the root nodules of legumes

Rhizobium is a biofertiliser. It forms a symbiotic association with the roots of leguminous plants such as pea, gram and beans. The roots develop small swellings called nodules, and the bacteria living inside the nodules fix atmospheric nitrogen into organic forms that the plant uses as a nutrient.

Option (1) In the leaves — Incorrect. Leaves are the food-making organs of the plant. Rhizobium does not live in leaves; it enters the plant through the roots.

Option (2) In the stems — Incorrect. Stems carry water and food and hold up the plant. No nitrogen-fixing nodules of Rhizobium form there.

Option (3) In the root nodules of legumes — Correct. Rhizobium lives inside the root nodules of legumes. In return for the nitrogen it fixes, the bacterium gets shelter and food from the plant.

Option (4) In the flowers — Incorrect. Flowers are reproductive organs. They have no link with nitrogen fixation by Rhizobium.

NCERT Extract:
Microbes as Biofertilisers – “… Biofertilisers are organisms that enrich the nutrient quality of the soil. The main sources of biofertilisers are bacteria, fungi and cyanobacteria. You have studied about the nodules on the roots of leguminous plants formed by the symbiotic association of Rhizobium. These bacteria fix atmospheric nitrogen into organic forms, which is used by the plant as nutrient. Other bacteria can fix atmospheric nitrogen while free-living in the soil (examples Azospirillum and Azotobacter), thus enriching the nitrogen content of the soil.”