During normal human knee kinematics, what dictates the obligatory posterior rollback of the femoral condyles during deep flexion, a crucial mechanism that posterior-stabilized (PS) total knee designs must replicate mechanically?

Explanation:
The correct option is c. In the native knee, the posterior cruciate ligament (PCL) provides the critical anatomical tether that forces the femoral condyles to roll posteriorly on the tibial plateau as the knee bends into deep flexion. This 'posterior rollback' clears the femur away from the posterior tibia to allow maximum flexion and significantly increases the lever arm of the extensor mechanism. In a Posterior-Stabilized (PS) TKA, the PCL is resected. To compensate, a polyethylene post on the tibial insert engages a metallic cam on the femoral component during flexion, mechanically forcing the femur to roll backward, replicating the function of the sacrificed PCL. Reference: Scott WN. Insall & Scott Surgery of the Knee.