Final answer:
The odontoid process is the projection on the axis (C2 vertebra) that interacts with the atlas (C1 vertebra) and enables head rotation. The correct option is a.
Step-by-step explanation:
The process that allows the pivot on the second cervical vertebrae, enabling us to turn our heads, is named the odontoid process. The second cervical (C2) vertebra, called the axis, is the pivot point for head rotation.
The axis has a distinctive bony projection known as the dens or odontoid process, which extends upward from the vertebral body. This process interacts with the first cervical vertebra (C1), also known as the atlas.
The odontoid process is held in place by the transverse ligament within the inner aspect of the atlas's anterior arch, facilitating the rotation of the head from side to side. The correct option is a.