`N_2` - fxing bacteria in legume plants are:

Options

A. Azotobacter
B. Frankia
C. Rizobia
D. None

Explanation

To answer the question regarding nitrogen-fixing bacteria in legume plants, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Nitrogen Fixation**:
- Nitrogen fixation is the process where atmospheric nitrogen (N₂) is converted into ammonia (NH₃), which plants can utilize.
- This process is essential because plants cannot directly use atmospheric nitrogen.

2. **Types of Nitrogen-Fixing Bacteria**:
- There are two main types of nitrogen-fixing bacteria:
- **Free-living Nitrogen-fixing Bacteria**: These bacteria exist independently in the soil and can fix nitrogen without a host plant.
- **Symbiotic Nitrogen-fixing Bacteria**: These bacteria form a symbiotic relationship with specific plants, particularly legumes, and fix nitrogen in association with them.

3. **Identifying Symbiotic Bacteria in Legumes**:
- The most notable symbiotic nitrogen-fixing bacteria associated with legume plants are **Rhizobium** species.
- Rhizobium forms nodules on the roots of legume plants, such as garden peas and sweet clover, where nitrogen fixation occurs.

4. **Evaluating the Options**:
- The question provides several options: Azotobacter, Frankia, Rhizobium, and none.
- **Azotobacter** is a free-living nitrogen-fixing bacterium and does not form symbiotic relationships with legumes.
- **Frankia** is also a nitrogen-fixing bacterium but primarily associates with non-leguminous plants.
- **Rhizobium** is the correct answer as it specifically forms a symbiotic relationship with legume plants.

5. **Conclusion**:
- Therefore, the nitrogen-fixing bacteria in legume plants are **Rhizobium**.

### Final Answer:
The nitrogen-fixing bacteria in legume plants are **Rhizobium**.

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