Final answer:
The lateral collateral ligament (LCL), also known as the fibular collateral ligament, is responsible for countering a medial force to the knee that results in a varus stress.
Step-by-step explanation:
The ligament that functions to counter a medial force to the knee, resulting in a varus stress, is the lateral collateral ligament (LCL). The LCL, also known as the fibular collateral ligament, is located on the lateral side of the knee. It spans from the lateral epicondyle of the femur to the head of the fibula. In the fully extended knee position, the LCL along with the medial collateral ligament (MCL), stabilizes and supports the knee by preventing side-to-side or rotational motions between the femur and tibia. Therefore, the correct answer to which ligament functions to resist a varus stress is A. Lateral collateral ligament.