Final answer:
The atlas (C1 vertebra) and the occipital condyles form the atlanto-occipital joint, which permits nodding the head 'yes'. The atlas is distinguished by its lack of a body and U-shaped curvature that allows the nodding motion. The axis (C2 vertebra) is responsible for rotational movements, like turning the head 'no'. The correct answer is option: d) Atlas
Step-by-step explanation:
The movement allowing the head to nod 'yes' is enabled by the articulation between the atlas (C1 vertebra) and the occipital condyles of the skull, forming the atlanto-occipital joint. This articulation permits considerable extension and flexion.
The atlas is unique compared to other cervical vertebrae as it does not have a body; it consists of an anterior and posterior arch. The U-shaped curvature of the superior articular processes of the atlas allows it to rock forward and backward on the occipital condyles, thus facilitating the nodding motion.
In contrast, the axis (C2 vertebra), with its distinctive dens (odontoid process), is crucial for rotation of the head, such as turning the head to say 'no'. The dens of the axis articulates with the atlas and is held in place by the transverse ligament, allowing for pivot-like rotational movements.