Select the correct statement regarding heterocysts.

Options

A. These are present in some filamentous cyanobacteria such as Nostoc and Anabaena.
B. These cells are specialized to perform `N_2` - fixation.
C. These cells contain enzyme nitrogenase.
D. All of these

Answer

### Step-by-Step Solution:

1. **Understanding Heterocysts**:
- Heterocysts are specialized cells found in certain filamentous cyanobacteria, such as Nostoc and Anabaena. They are typically larger, pale, and yellowish in color compared to the surrounding vegetative cells.

2. **Function of Heterocysts**:
- The primary function of heterocysts is nitrogen fixation. They convert atmospheric nitrogen (N₂) into ammonia (NH₃), which can be utilized by the organism for growth and development.

3. **Location and Structure**:
- Heterocysts are interspersed among vegetative cells in the filamentous structure of these bacteria. While vegetative cells are involved in photosynthesis and food production, heterocysts are dedicated to nitrogen fixation.

4. **Enzyme Involvement**:
- The enzyme responsible for nitrogen fixation is nitrogenase. This enzyme is present in heterocysts and is crucial for the conversion of nitrogen gas into a usable form.

5. **Oxygen Permeability**:
- Heterocysts are adapted to be impermeable to oxygen. This is essential because the nitrogenase enzyme is inhibited by oxygen. However, they are permeable to nitrogen, allowing the uptake of atmospheric nitrogen for fixation.

6. **Conclusion**:
- Based on the above points, all the statements regarding heterocysts are correct. Therefore, the correct answer to the question is that all options provided (1, 2, and 3) are true.