Nostoc fixes nitrogen in the symbiotic association of the following
1. Alnus
2. Gunnera
3. Anthoceros
4. Casuarina.
The correct combination is

Options

A. 1 and 2
B. 1 and 3
C. 1 and 4
D. 2 and 3.

Answer

To solve the question regarding the symbiotic association of Nostoc in nitrogen fixation, follow these steps:

### Step-by-Step Solution:

1. **Understand the Organism Nostoc**:
- Nostoc is a type of blue-green algae, also known as cyanobacteria, which has the ability to fix atmospheric nitrogen. This means it can convert nitrogen gas from the atmosphere into a form that can be used by plants.

**Hint**: Remember that Nostoc is a cyanobacterium known for nitrogen fixation.

2. **Identify the Role of Heterocysts**:
- Nostoc contains specialized cells called heterocysts. These cells are crucial for the process of nitrogen fixation, as they provide an anaerobic environment necessary for the enzyme nitrogenase to function.

**Hint**: Heterocysts are the key cells in Nostoc that facilitate nitrogen fixation.

3. **Analyze the Given Options**:
- The question provides four options: Alnus, Gunnera, Anthoceros, and Casuarina. We need to determine which of these plants form a symbiotic relationship with Nostoc for nitrogen fixation.

4. **Evaluate Each Option**:
- **Alnus**: This is a genus of trees known to associate with different types of nitrogen-fixing bacteria, but not specifically with Nostoc.
- **Gunnera**: This plant is known to form a symbiotic relationship with Nostoc for nitrogen fixation.
- **Anthoceros**: This is a genus of hornworts that also has a symbiotic relationship with Nostoc for nitrogen fixation.
- **Casuarina**: Similar to Alnus, Casuarina does not specifically associate with Nostoc for nitrogen fixation.

**Hint**: Focus on which plants are known to have a symbiotic relationship with Nostoc.

5. **Conclusion**:
- Based on the evaluation, the correct options are Gunnera and Anthoceros. Therefore, the correct combination of organisms that Nostoc fixes nitrogen in symbiotic association with is **Gunnera and Anthoceros**.

### Final Answer:
The correct combination is **Gunnera (2) and Anthoceros (3)**.