I - Statement. Cyanobacteria do nitrogen fixation.
II- Explanation. All cyanobacteria possess heterocyst.

Options

A. I and II both correct
B. I and II both wrong
C. I is correct, II is wrong
D. I is wrong , II os correct.

Answer

### Step-by-Step Solution:

1. **Understand the Statement**: The first part of the question states, "Cyanobacteria do nitrogen fixation." This statement is true. Cyanobacteria are known for their ability to fix atmospheric nitrogen into a usable form for plants.

2. **Analyze the Explanation**: The second part of the question states, "All cyanobacteria possess heterocyst." This statement is false. While some cyanobacteria, such as Nostoc and Anabaena, do possess heterocysts (specialized cells for nitrogen fixation), not all cyanobacteria have this feature.

3. **Evaluate the Options**:
- **Option 1**: Both statements are correct. (Incorrect, because the explanation is false)
- **Option 2**: Both statements are wrong. (Incorrect, because the statement is true)
- **Option 3**: The first statement is correct, but the second is wrong. (Correct)
- **Option 4**: The first statement is wrong, but the second is correct. (Incorrect, because the statement is true)

4. **Conclusion**: The correct option is **Option 3**, which states that the first statement is correct, but the second statement is wrong.