Final answer:
The structure closest to the posterior capsule of the knee is the posterior cruciate ligament.
Step-by-step explanation:
The structure closest to the posterior capsule of the knee is the posterior cruciate ligament. It extends from the posterior, superior surface of the tibia to the inner aspect of the medial condyle of the femur. The posterior cruciate ligament prevents anterior displacement of the femur when the knee is flexed and weight bearing, providing support to the knee joint.