Pivot joint is
(a) Atlanto-axial joint
(b) Shoulder joint
(c) Hip joint
(d) None

Options

A. Atlanto-axial joint
B. Shoulder joint
C. Hip joint
D. None

Answer

To determine which of the given joints is a pivot joint, we can analyze each option step by step:

1. **Understanding the Types of Joints**:
- Joints are classified into several types based on their structure and movement capabilities. The main types include ball and socket joints, hinge joints, pivot joints, and others.

2. **Analyzing the Options**:
- **Option (a): Atlanto-axial joint**: This joint is located in the neck and connects the first cervical vertebra (atlas) to the second cervical vertebra (axis). It allows for rotation of the head.
- **Option (b): Shoulder joint**: This is a ball and socket joint that allows for a wide range of motion in the arm.
- **Option (c): Hip joint**: Similar to the shoulder, the hip joint is also a ball and socket joint, allowing movement in multiple directions.
- **Option (d): None**: This option implies that none of the joints listed are pivot joints.

3. **Identifying the Pivot Joint**:
- A pivot joint allows for rotational movement around a single axis. The atlanto-axial joint is specifically designed for this type of movement, enabling the head to turn from side to side.

4. **Conclusion**:
- Since the shoulder and hip joints are ball and socket joints and do not fit the definition of a pivot joint, the only correct answer is **Option (a): Atlanto-axial joint**.

### Final Answer:
The correct answer is **(a) Atlanto-axial joint**.