Final answer:
The ilium of the coxal bone articulates with the sacrum at the auricular surface, forming the sacroiliac joint.
Step-by-step explanation:
The region of the coxal bone that articulates with the sacrum is the ilium. Specifically, it is the auricular surface of the ilium that forms the joint with the sacrum, known as the sacroiliac joint. The articulation between the auricular surfaces of the sacrum and ilium provides a strong connection that supports the weight of the upper body and allows for limited motion which is crucial for shock absorption and balance during movement.