Answer:
a. the sartorius
Step-by-step explanation:
This muscle is the one that can move the knee joint and the hip joint, for this reason, is the one that allows to pass the heel form one foot in front of the knee of the other foot.
The function of the sartorius muscles to allow to cross the leg over the other is because is the longest muscle in our body. Because of it position and length, this muscle allow to rotate, flex and adduct the hip.
I add an image, so you can imagine the position of the muscle in our body.
Hope this info is useful.