Polymorphism occurs in

Options

A. Anthozoa
B. Scyphozoa
C. Rhizopoda
D. Hydrozoa.

Answer

### Step-by-Step Solution:

1. **Understanding Polymorphism**:
- Polymorphism refers to the occurrence of different forms or stages in the life cycle of an organism. In the context of cnidarians, it typically involves two distinct body forms.

2. **Identifying the Forms in Cnidarians**:
- Cnidarians exhibit two main body forms:
- **Polyp**: This is a sessile (non-moving) form, which is cylindrical in shape. Examples include Hydra and Adamasia.
- **Medusa**: This is a free-swimming form, often umbrella-shaped. An example is Aurelia (the moon jellyfish).

3. **Explaining Alternation of Generations**:
- In cnidarians, these two forms (polyp and medusa) can alternate in their life cycle. This phenomenon is known as alternation of generations, where one form reproduces to produce the other form.

4. **Focusing on Hydrozoa**:
- Hydrozoa is a class within the phylum Cnidaria that prominently exhibits polymorphism. Members of Hydrozoa can exist in both polyp and medusa forms, showcasing the characteristic of polymorphism.

5. **Conclusion**:
- Based on the above information, we can conclude that polymorphism occurs in the class Hydrozoa within the phylum Cnidaria.

### Final Answer:
- Polymorphism occurs in **Hydrozoa** (Option 4).

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