Scyphozoan medusa is commonly known as
(a) Sea Pen
(b) Jelly Fish
(c) Obelia
(d) Sea Urchin

Options

A. Sea Pen
B. Jelly Fish
C. Obelia
D. Sea Urchin.

Answer

To solve the question "Scyphozoan medusa is commonly known as," we will analyze the options provided and identify the correct one based on our knowledge of marine organisms.

### Step-by-Step Solution:

1. **Understanding Scyphozoan Medusa**:
- Scyphozoans are a class of marine animals within the phylum Cnidaria. They are commonly referred to as jellyfish. The medusa form is the dominant stage in their life cycle.

2. **Analyzing the Options**:
- **Option (a) Sea Pen**: Sea pens are a type of colonial cnidarian but are not classified as scyphozoans. They are part of the order Pennatulacea.
- **Option (b) Jelly Fish**: This is the common name for scyphozoans. They are characterized by their gelatinous bodies and umbrella-shaped bells.
- **Option (c) Obelia**: Obelia is a genus of colonial hydrozoans, not scyphozoans.
- **Option (d) Sea Urchin**: Sea urchins are echinoderms and are not related to jellyfish or scyphozoans.

3. **Identifying the Correct Answer**:
- Based on the definitions and classifications, the correct answer is **(b) Jelly Fish**. Scyphozoan medusa is indeed commonly known as jellyfish.

4. **Conclusion**:
- The common name of Scyphozoan medusa is jellyfish, making option (b) the correct choice.

### Final Answer:
(b) Jelly Fish