Explanation:
The correct option is 'a'. The 'spur sign' is seen on the obturator oblique view and represents the intact portion of the ilium attached to the sacroiliac joint, projecting medially to the displaced posterior column. It is a pathognomonic finding for a both-column fracture in the Letournel classification system, indicating complete dissociation of the articular surface from the axial skeleton.

Reference: Letournel E. Acetabulum fractures: classification and management. Clin Orthop Relat Res. 1980;(151):81-106.