A colonial hydrozoan is

Options

A. Hydra
B. Metridium
C. Aurelia
D. Obelia.

Answer

To solve the question "A colonial hydrozoan is," we need to analyze the options provided: Hydra, Matridium, Aurelia, and Obelium.

### Step-by-Step Solution:

1. **Understanding Hydrozoa**:
- Hydrozoa is a class within the phylum Cnidaria, which includes both solitary and colonial organisms.

2. **Identifying Colonial Hydrozoans**:
- Colonial hydrozoans are those that live in colonies, where individual organisms (polyps) are connected and function together.

3. **Analyzing the Options**:
- **Hydra**: This is primarily a solitary organism and does not form colonies.
- **Matridium**: This is a type of sea anemone and is also not classified as a hydrozoan.
- **Aurelia**: This is a jellyfish, which is a medusa and not a colonial organism.
- **Obelium**: This is recognized as a colonial hydrozoan that exhibits both polyp and medusa stages in its life cycle.

4. **Conclusion**:
- Based on the definitions and characteristics of the organisms listed, the correct answer is **Obelium**, as it is a colonial hydrozoan.

### Final Answer:
A colonial hydrozoan is **Obelium**.