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**: Start by recognizing that cyanobacteria are a type of bacteria that can perform photosynthesis. They are often referred to as blue-green algae, although they are not true algae.

2. **Classification**: Cyanobacteria belong to the kingdom Monera. This kingdom includes prokaryotic organisms, which are primarily unicellular.

3. **Cell Structure**: It is important to note that cyanobacteria are unicellular organisms. While they can form colonies or filaments, they are not multicellular organisms.

4. **Habitat**: Cyanobacteria can be found in various environments. They are not limited to marine habitats; they can also be found in freshwater and terrestrial environments. This makes the statement that they are "freshwater, marine, or terrestrial organisms" correct.

5. **Nutritional Mode**: Cyanobacteria are primarily photosynthetic autotrophs, meaning they can produce their own food using sunlight. They contain chlorophyll a, which is similar to that found in higher plants. However, they do not fall under the category of chemosynthetic autotrophs, which obtain energy by oxidizing inorganic substances.

6. **Conclusion**: Based on the analysis of the options provided, the correct statement about cyanobacteria is that they are freshwater, marine, or terrestrial organisms.

**Final Answer**: Cyanobacteria are freshwater, marine, or terrestrial organisms.

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