Explanation:
The correct option is 'd'. A Jefferson fracture is a burst fracture of the atlas (C1 ring), typically involving fractures through the anterior and posterior arches. It is classically caused by an axial loading force to the vertex of the skull, such as diving into a shallow pool or an object falling directly onto the head, driving the occipital condyles downward and forcing the lateral masses of C1 outward.

Reference: Jefferson G. Fracture of the atlas vertebra: report of four cases, and a review of those previously recorded. Br J Surg. 1920;7:407-422.