Final answer:
For irregular shaped bones, there is no specific reference for the direction of fracture (D) as these bones have complex shapes without a defined longitudinal axis, unlike long bones.
Step-by-step explanation:
The reference for the direction of fracture for irregular shaped bones is D) No specific reference for irregular bones. Irregular bones, such as the vertebrae, hip bones, and several skull bones, have complex shapes that include short, flat, notched, or ridged surfaces, making it difficult to apply a single reference axis for the fracture direction. These bones do not have a defined longitudinal axis like long bones do, which are longer than they are wide and include a shaft and two ends. Instead, fractures in irregular bones are typically described based on the specific bone's anatomy and the nature of the fracture.