Answer;
Ilium.
Ilium is the region of the coxal bone that articulates with the sacrum.
Explanation;
The hip bones are made of paired bones, one either side of the body, namely ilium, ischium, and pubis.
Ilium is the largest and uppermost bone of the pelvis and accounts for the width of the pelvis.
Each ilium connects posteriorly with the sacrum at a sacroiliac joint.
