Final answer:
The bone feature unique to cervical vertebrae is the transverse foramen, which is an opening in each transverse process allowing passage of an artery to the brain.
Step-by-step explanation:
The bone feature only found on cervical vertebrae is d. a transverse foramen. Typical cervical vertebrae, like C4 or C5, are characterized by several distinct features that differentiate them from thoracic or lumbar vertebrae. Most notably, each transverse process of the cervical vertebrae has an opening called the transverse foramen, through which an important artery that supplies the brain ascends up the neck. This feature is unique to cervical vertebrae and is not found in the transverse processes of thoracic or lumbar vertebrae. Other features of cervical vertebrae include a small body, a bifid (Y-shaped) spinous process, and articular processes that largely face upward or downward.