Select the mismatch:

Options

A. Frankia - Alnus
B. Rhodospirillum - Mycorrhiza
C. Anabaena - Nitrogen fixer
D. Rhizobium - Alfalfa

Answer

To solve the question of selecting the mismatch, we need to analyze each option provided and determine whether the associations mentioned are correct.

1. **Frankia and Alnus**:
- Frankia is a gram-positive filamentous bacterium that forms a symbiotic relationship with the genus Alnus (alders). This association is known to be correct.
- **Conclusion**: This pair is correctly matched.

2. **Rhodospirilium**:
- Rhodospirilium is an anaerobic, free-living microbe that is not typically associated with mycorrhiza. Mycorrhiza refers to a symbiotic association between fungi and plant roots, which is a different type of relationship.
- **Conclusion**: This pair is mismatched.

3. **Anabaena**:
- Anabaena is a genus of nitrogen-fixing cyanobacteria that can form symbiotic relationships with certain plants, such as water ferns. This association is correct.
- **Conclusion**: This pair is correctly matched.

4. **Rhizobium**:
- Rhizobium is a genus of bacteria that forms a symbiotic relationship with legumes, helping them fix nitrogen in the soil. This association is also correct.
- **Conclusion**: This pair is correctly matched.

After analyzing all the options, we find that the mismatch is between **Rhodospirilium and mycorrhiza**.

### Final Answer:
The mismatch is **Rhodospirilium and mycorrhiza**.