**Core Concept**
The anterior cruciate ligament (ACL) is a critical stabilizing structure in the knee joint, responsible for preventing excessive forward movement of the tibia relative to the femur. It plays a crucial role in maintaining knee stability during various activities such as pivoting, twisting, and deceleration.

**Why the Correct Answer is Right**
The ACL prevents anterior tibial translation, which is the forward movement of the tibia in relation to the femur. This is achieved through the ACL's tautness and its attachment to the anterior horns of the lateral and medial femoral condyles and the posterior horns of the tibia. The ACL works in conjunction with the posterior cruciate ligament (PCL) to maintain the normal kinematics of the knee joint. When the ACL is injured or deficient, this can lead to a variety of knee pathologies, including instability, meniscal tears, and osteoarthritis.

**Why Each Wrong Option is Incorrect**
**Option A:** The ACL does not prevent varus or valgus stress, which is the inward or outward movement of the knee joint. This is the function of the medial and lateral collateral ligaments.

**Option B:** The ACL does not prevent posterior tibial translation, which is the backward movement of the tibia in relation to the femur. This is the function of the posterior cruciate ligament (PCL).

**Option C:** The ACL does not prevent internal or external rotation of the tibia. This is the function of the ligaments and muscles surrounding the knee joint, including the medial and lateral collateral ligaments and the popliteus muscle.

**Clinical Pearl / High-Yield Fact**
The ACL is commonly injured during sports activities that involve sudden changes of direction, pivoting, or landing from jumps. Patients with ACL injuries often present with a history of a sudden, severe twisting motion, followed by a "pop" or "snap" sensation in the knee.

**Correct Answer: D.**