Cyanobacteria are :

Options

A. mostly multicellular, colonial and marine
B. filamentous and marine always
C. fresh water, marine or terrestrial organisms
D. photosynthetic and chemosynthetic autotrophs

Answer

**Step-by-Step Solution:**

1. **Understanding Cyanobacteria**: Begin by identifying what cyanobacteria are. They are a group of photosynthetic bacteria commonly known as blue-green algae.

2. **Analyzing Options**:
- **Option 1**: "Cyanobacteria are mostly multicellular, colonial and marine."
- This option is incorrect because cyanobacteria are primarily unicellular, although some can form colonies.

- **Option 2**: "Filamentous and marine always."
- This option is also incorrect. While some cyanobacteria can be filamentous, they are not exclusively marine; they can also be found in freshwater and terrestrial environments.

- **Option 3**: "Freshwater, marine and terrestrial organisms."
- This option is correct. Cyanobacteria can thrive in a variety of habitats including freshwater, marine, and terrestrial environments.

- **Option 4**: "Photosynthetic, chemosynthetic autotrophs."
- This option is misleading. While cyanobacteria are indeed photosynthetic autotrophs, they are not chemosynthetic. Therefore, this option is incorrect.

3. **Conclusion**: Based on the analysis, the correct answer is **Option 3: Freshwater, marine and terrestrial organisms**.

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