The bacterium, found in root nodules of leguminous plants is kwon as:

Options

A. Anabaena
B. Nostoc
C. Azotobacter
D. Rhizobium

Explanation

**Step-by-Step Solution:**

1. **Identify the Context**: The question asks about a specific bacterium found in the root nodules of leguminous plants.

2. **Understand the Role of Bacteria**: In leguminous plants, there are symbiotic bacteria present in the root nodules. Symbiotic means that both the plant and the bacteria benefit from this relationship.

3. **Recognize the Function of the Bacteria**: These bacteria are involved in nitrogen fixation. They convert atmospheric nitrogen into a form that the plants can use, which enriches the soil with nitrogen.

4. **Identify the Specific Bacterium**: The specific bacterium that is commonly found in the root nodules of leguminous plants is known as *Rhizobium*.

5. **Summarize the Relationship**: In summary, *Rhizobium* bacteria live in the root nodules of leguminous plants, providing them with nitrogen while receiving food and shelter in return.

**Final Answer**: The bacterium found in root nodules of leguminous plants is known as *Rhizobium*.

---