Final answer:
The cup-shaped socket of the hip bone into which the thigh bone fits is called the acetabulum. It is formed by the ilium, pubis, and ischium and articulates with the head of the femur to form the hip joint.
Step-by-step explanation:
The cup-shaped socket of the hip bone into which the thigh bone fits is called the acetabulum. The acetabulum is a deep, cup-shaped cavity that is located on the lateral side of the hip bone and is formed by the junction of the three areas of each hip bone: the ilium, pubis, and ischium. This cavity articulates with the rounded head of the femur, or thigh bone, to form the hip joint.