Coenobium is the colony of:-

Options

A. Volvox
B. Spirogyra
C. Ulothrix
D. Acetabularia

Answer

### Step-by-Step Solution:

1. **Understanding Coenobium**: Coenobium refers to a specific type of colony formation in certain algae. It is characterized by a fixed number of cells that are often embedded in a mucilaginous matrix.

2. **Identifying the Organisms**: The question provides four options: volvox, spirogyra, eulothrix, and acetabularia. We need to determine which of these forms a coenobium.

3. **Analyzing Each Option**:
- **Volvox**: This is a green algae that forms a motile colony with a definite shape and a fixed number of cells. The colony can range from 500 to 60,000 cells, making it a classic example of coenobium.
- **Spirogyra**: This is a filamentous green algae known for its spiral arrangement of chloroplasts. It does not form coenobium.
- **Eulothrix**: This is a non-branching filamentous green algae that exists in both freshwater and saltwater. It also does not form coenobium.
- **Acetabularia**: This is a genus of green algae that is typically unicellular and does not form colonies like coenobium.

4. **Conclusion**: Based on the analysis, the only organism that forms a coenobium is **Volvox**.

5. **Final Answer**: Therefore, the correct answer to the question "Coenobium is the colony of:" is **Volvox**.