Most students know that the femoral head is convex and the acetabulum is concave without obtaining that information from the problem. Because this problem specifies open chain hip flexion, we understand that the femur is the moving segment. We refer to the rules of concavity and convexity to learn that the moving femur must roll and glide in opposite directions. Finally, we visualize the femoral head rolling anteriorly on the stable acetabular surface during hip flexion. When the femur rolls anteriorly on the acetabulum, it must glide posteriorly to maintain the hip's integrity. Response 'b' describes this movement.

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