Bacterium associated with legume roots is

Options

A. Rhizobium
B. Nostoc
C. Spirogyra
D. Clostridium.

Answer

### Step-by-Step Solution:

1. **Understand the Question**: The question asks for the bacterium that is associated with legume roots. This indicates we are looking for a specific type of bacteria that forms a relationship with leguminous plants.

2. **Evaluate the Options**:
- **Option A: Rhizobium**: This is known to be a nitrogen-fixing bacterium that forms a symbiotic relationship with leguminous plants, specifically in their root nodules.
- **Option B: Nostoc**: This is a cyanobacterium, not a true bacterium, and while it can fix nitrogen, it does not specifically associate with legume roots.
- **Option C: Spirogyra**: This is a green alga, not a bacterium, and does not play a role in nitrogen fixation in leguminous plants.
- **Option D: Clostridium**: This is a nitrogen-fixing bacterium, but it is free-living and does not form a symbiotic relationship with legume roots.

3. **Identify the Correct Answer**: Based on the evaluation, Rhizobium is the only bacterium that is specifically associated with legume roots and forms a symbiotic relationship with them. It helps in nitrogen fixation, providing essential nutrients to the plant.

4. **Conclusion**: The correct answer to the question is **Rhizobium**.