Final answer:
The left intervertebral foramina are best demonstrated on a right posterior oblique (RPO) cervical spine X-ray due to the angle at which the body is positioned, allowing clearer visualization of these structures.
Step-by-step explanation:
The anatomical structure that is best demonstrated on a right posterior oblique (RPO) cervical spine X-ray is the left intervertebral foramina. In an RPO position, the patient's body is rotated such that the right side is posterior and the left side is anterior. This angle opens the view of the intervertebral foramina on the side opposite to the direction of obliquity. Therefore, the left intervertebral foramina can be visualized more clearly. These foramina are the passageways through which spinal nerves exit from the vertebral column. Each typical cervical vertebra has a transverse process with a transverse foramen for the passage of these important nerves, and the positions of the vertebrae create the intervertebral foramen between adjacent vertebrae.