In which of the following class of phylum mollusca, torsion occurs?

Options

A. Gastropoda
B. Pelecypoda
C. Cephalopoda
D. Monoplacophora

Explanation

To solve the question "In which of the following class of phylum Mollusca, torsion occurs?", we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Concept of Torsion**:
- Torsion is a developmental process that occurs in certain mollusks, leading to a twisting of the body. This results in the positioning of the anus above the mouth, which is a unique characteristic.

2. **Identify the Classes of Phylum Mollusca**:
- The main classes of the phylum Mollusca include:
- Gastropoda
- Bivalvia (or Pelecypoda)
- Cephalopoda
- Monoplacophora
- Polyplacophora

3. **Determine Which Class Exhibits Torsion**:
- Among these classes, torsion is primarily associated with **Gastropoda**. This class includes snails and slugs, which are known for this characteristic.

4. **Evaluate Other Classes**:
- **Bivalvia (Pelecypoda)**: This class includes clams and oysters, which do not exhibit torsion.
- **Cephalopoda**: This class includes octopuses and squids, which also do not undergo torsion in the same way as gastropods.
- **Monoplacophora**: This class is not known for torsion either.

5. **Conclude the Answer**:
- Based on the information gathered, the correct answer to the question is **Gastropoda**, as torsion occurs in this class of the phylum Mollusca.

### Final Answer:
**Torsion occurs in the class Gastropoda of phylum Mollusca.**

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