Final answer:
The bone that articulates with the dens is the atlas (A). This articulation allows the atlas to rotate on top of the axis, moving the head toward the right or left, as when shaking your head "no."
Step-by-step explanation:
The bone that articulates with the dens is the atlas (A). The dens is a bony projection that extends upward from the vertebral body of the axis (B). It joins with the inner aspect of the anterior arch of the atlas, where it is held in place by the transverse ligament. This articulation allows the atlas to rotate on top of the axis, moving the head toward the right or left, as when shaking your head "no."