The answer is C. Medial collateral ligament (MCL). The MCL is located on the inner side of the knee joint and provides stability to the knee by resisting valgus stress, which is a force that pushes the knee inward towards the opposite leg. It also helps to resist external rotational forces, which can occur when the foot is planted on the ground and the body twists. The anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL) are located in the center of the knee joint and help to stabilize the knee from front-to-back movements. The lateral collateral ligament (LCL) is located on the outer side of the knee joint and helps to stabilize the knee against varus stress, which is a force that pushes the knee outward away from the opposite leg.