What action involves rotating the head from side to side?

The action of rotating the head from side to side is correctly identified as rotation. In a rotational movement, a body part turns around its own axis. When applied to the neck and head, this rotation allows for lateral movement, enabling a person to look to the left and right.

When considering the other options, tilting generally refers to the lateral bending of the head toward the shoulder, which does not involve the complete rotation. Flexion involves decreasing the angle between body parts, such as bending the neck forward, and extension is straightening a bent part, such as looking upwards. Both of these movements do not pertain to the side-to-side motion described in the question. Thus, the action that specifically describes the side-to-side movement of the head is rotation.