Motile coenobium occurs in

Options

A. Ulva
B. Palmella
C. Volvox
D. Hydrodictyon.

Answer

### Step-by-Step Solution:

1. **Understanding the term "coenobium":**
- A coenobium refers to a colony of unicellular organisms that are loosely associated and may be surrounded by a common membrane. This structure is often found in certain types of algae.

2. **Identifying the nature of coenobia:**
- Coenobia can be motile or non-motile. The motility of these colonies depends on the type of organism and its environmental adaptations.

3. **Recognizing examples of coenobia:**
- In the context of algae, coenobia can be found in various forms. For instance, the motile coenobium is exemplified by the genus **Hydrodicton**. This genus consists of green algae that form motile colonies.

4. **Differentiating between motile and non-motile coenobia:**
- While Hydrodicton is a motile coenobium, another example of a non-motile coenobium is **Volvox**, which is also a type of green algae but does not exhibit the same level of motility.

5. **Conclusion:**
- Based on the understanding of coenobia and the examples discussed, the correct answer to the question "Motile coenobium occurs in" is **Hydrodicton**.

### Final Answer:
The motile coenobium occurs in **Hydrodicton**.

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