Final answer:
"During the extension, the femur slides backwards on the tibia (Option D)" is the true statement. It occurs with a slight medial rotation to lock the knee in place. When flexing the knee, a slight lateral rotation of the femur is initiated by the popliteus muscle to unlock the knee.
Step-by-step explanation:
The correct statement concerning the knee joint during its motion is that during flexion and extension the femur both rolls and glides on the tibia to maintain stable contact between the bones in all knee positions.
Specifically, when the knee extends, the femur slides backwards on the tibia after the lateral condyle of the femur finishes its rolling motion before the larger medial condyle.
This results in a slight medial rotation that helps to "lock" the knee in its fully extended and most stable position.
Conversely, during flexion, a slight lateral rotation of the femur on the tibia, the action required to "unlock" the knee, is initiated by the popliteus muscle.
Given the description of the knee mechanics, we can conclude that both statements A and B are incorrect because they suggest that the femur must slide forward on the tibia during flexion to prevent rolling off.
However, when the knee is in flexion, the articulations allow for some rotation, and it is not merely a forward slide that prevents the femur from rolling off.
Statement D is correct because it accurately describes the sliding backwards of the femur on the tibia during extension, contributing to knee stability.
Therefore, the correct answer to the question is Option D: "During extension, the femur must slide backward on the tibia."