The microbe that produces root nodules on the root of Alnus is

Options

A. Nitrobacter
B. Rhizobium
C. Nitrosomonas
D. Frankia

Answer

To solve the question regarding the microbe that produces root nodules on the roots of Alnus, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Question**: We need to identify which microbe forms root nodules on Alnus. This indicates a symbiotic relationship between the microbe and the plant.

2. **Identify the Options**: The options provided include various bacteria. We need to determine which of these bacteria is known for forming nodules specifically on Alnus.

3. **Evaluate Symbiotic Relationships**:
- **Rhizobium**: This bacterium is known for forming nodules on leguminous plants. Since Alnus is not a legume, Rhizobium cannot be the correct answer.
- **Francia**: This bacterium is known to form nodules on the roots of non-leguminous plants, including Alnus. Therefore, it is a candidate for the correct answer.
- **Nitrosomonas and Nitrobacter**: Both of these bacteria are free-living nitrogen-fixing bacteria and do not form nodules on plant roots. Thus, they are not the correct answers.

4. **Conclusion**: Based on the evaluation, the microbe that produces root nodules on the root of Alnus is **Francia**.

### Final Answer:
The microbe that produces root nodules on the root of Alnus is **Francia**.