The acetabular fossa is a depression in the acetabulum, which is a part of the hip bone. It is located on the inner surface of the acetabulum and is important for providing stability and support to the hip joint. The acetabulum, on the other hand, is a cup-shaped socket that forms the hip joint with the head of the femur. It is responsible for articulating with the femur and allowing for movement of the hip joint. Therefore, the correct answer is "Acetabular fossa, acetabulum."