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

Options

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

Answer

### Step-by-Step Text Solution:

1. **Understanding the Assertion**: The assertion states that "Anabaena is commonly called a symbiotic biofertiliser." This means that Anabaena plays a role in enhancing soil fertility through its symbiotic relationships.

2. **Understanding the Reason**: The reason given is "Anabaena is found as an endosymbiont in the thallus of Anthoceros." This indicates that Anabaena lives inside the body of Anthoceros (a type of hornwort) and contributes to nitrogen fixation.

3. **Evaluating the Assertion**: Anabaena is indeed known for its ability to fix nitrogen, which makes it a beneficial organism for agriculture and soil health. It forms symbiotic relationships with various plants, including the water fern Azolla, and is recognized as a biofertilizer.

4. **Evaluating the Reason**: The reason is also accurate. Anabaena is found as an endosymbiont in the thallus of Anthoceros, where it helps in nitrogen fixation. This relationship is mutually beneficial, as the plant provides a safe environment for Anabaena, while Anabaena supplies the plant with nitrogen.

5. **Conclusion**: Both the assertion and the reason are true. Furthermore, the reason correctly explains the assertion, as the symbiotic relationship between Anabaena and Anthoceros is what allows Anabaena to function as a biofertilizer.

6. **Final Answer**: Therefore, the correct answer is that both the assertion and the reason are true, and the reason is the correct explanation of the assertion.