Name the best known symbiotic nitrogen fixing bacterium.

Explanation

**Step-by-Step Solution:**

1. **Understanding Nitrogen Fixation**: Nitrogen fixation is the process by which certain bacteria convert atmospheric nitrogen (N₂) into ammonia (NH₃) or related compounds that plants can use. This is crucial for the nitrogen cycle and for plant growth.

2. **Identifying Nitrogen-Fixing Bacteria**: There are several types of nitrogen-fixing bacteria, which can be categorized into free-living and symbiotic bacteria. Symbiotic nitrogen-fixing bacteria form a mutual relationship with plants, particularly legumes.

3. **Focusing on Symbiotic Bacteria**: Among the symbiotic nitrogen-fixing bacteria, one of the most well-known is Rhizobium. This bacterium forms nodules on the roots of leguminous plants, where it fixes nitrogen.

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