Explanation:

The nodding movement of the head occurs due to the joint between the Atlas (C1 vertebra) and the occipital condyles of the skull. This joint is known as the atlanto-occipital joint.

Atlas and Occipital Condyles:
- The Atlas is the first cervical vertebra, located at the top of the vertebral column. It is unique in structure as it lacks a body and has a ring-like shape.
- The occipital condyles are two oval-shaped projections on the occipital bone of the skull. They articulate with the Atlas to form the atlanto-occipital joint.

Function of the Atlanto-occipital Joint:
- The atlanto-occipital joint primarily allows the nodding or flexion-extension movements of the head.
- The joint permits the head to move forward and backward in a "yes" motion, as well as some degree of lateral flexion.
- This joint is responsible for the up-and-down movement of the head, allowing us to nod.

Mechanism of Nodding:
- The Atlas has a concave facet on its superior surface that articulates with the convex occipital condyles.
- This joint is highly specialized, allowing for a wide range of flexion and extension movements.
- When we nod, the Atlas tilts forward, causing the occipital condyles to slide forward and downward on the superior facets of the Atlas.
- This movement is facilitated by the presence of synovial fluid within the joint, which reduces friction.
- The nodding movement of the head is controlled by the contraction and relaxation of the muscles surrounding the atlanto-occipital joint.

Conclusion:
The correct answer is option B, as the nodding movement of the head occurs due to the joint between the Atlas (C1 vertebra) and the occipital condyles of the skull, known as the atlanto-occipital joint. This joint allows for flexion-extension movements of the head, enabling the up-and-down nodding motion.