Assertion : Anabaena is commonly called a symbiotic biofertilizer.
Reason: Anabaena is found as endosymbiont in the thallus of Anthoceras.

Options

A. If both Assertion and Reason are true and Reason is a correct explanation of Assertion.
B. If both Assertion and Reason are true but Reason is not a correct explanation of Assertion
C. It Assertion and true but Reason is false.
D. If both Assertion and Reason are false.

Answer

### Step-by-Step Solution:

1. **Understanding the Assertion**: The assertion states that "Anabaena is commonly called a symbiotic biofertilizer." This is true because Anabaena, a type of cyanobacteria, forms symbiotic relationships with certain plants, particularly in providing them with nitrogen, which is essential for their growth.

2. **Understanding the Reason**: The reason given is that "Anabaena is found as an endosymbiont in the thallus of Anthoceros." This statement is incorrect. Anabaena is not found in Anthoceros; rather, it is found in association with a different plant, such as Azolla, which is a water fern.

3. **Evaluating the Relationship**: Anabaena provides nitrogen to its host plant through nitrogen fixation, which is crucial for the plant's nutrient supply. In return, the plant offers shelter and nutrients to Anabaena. This mutualistic relationship is a characteristic of biofertilizers.

4. **Conclusion**: Since the assertion is true and the reason is false, we conclude that the correct answer is that the assertion is true, but the reason is false.

### Final Answer:
- Assertion: True
- Reason: False