The phenomenon of torsion occurs in :

Options

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

Answer

To solve the question regarding the phenomenon of torsion, we will analyze each option provided in the context of the phenomenon.

### Step-by-Step Solution:

1. **Understanding Torsion**:
- Torsion is a developmental process observed in certain mollusks where the visceral mass twists 180 degrees relative to the head and foot. This results in the repositioning of the anus and mantle cavity above the head, allowing for better protection and mobility.

2. **Evaluating the Options**:
- **Option 1: Gastropod**:
- Gastropods are known for undergoing torsion during their larval development. This is the correct answer as the torsion phenomenon is a characteristic feature of this class of mollusks.

- **Option 2: Pallisiopoda**:
- This class includes marine and freshwater mollusks that do not exhibit torsion. Therefore, this option is incorrect.

- **Option 3: Cephalopoda**:
- Cephalopods, such as squids and octopuses, have a different body plan and do not undergo torsion. They exhibit bilateral symmetry and have a prominent head. Thus, this option is also incorrect.

- **Option 4: Amphinous**:
- Amphinous refers to a group of marine mollusks that do not exhibit torsion. This option is incorrect as well.

3. **Conclusion**:
- After evaluating all options, the only class that exhibits the phenomenon of torsion is **Gastropoda**. Therefore, the correct answer is **Option 1: Gastropod**.

### Final Answer:
The phenomenon of torsion occurs in **Gastropod**.