Which muscle functions to rotate the leg laterally?

The biceps femoris is the muscle that primarily functions to rotate the leg laterally. This muscle is one of the hamstrings and is located on the posterior aspect of the thigh. Its role in external rotation becomes particularly evident when the knee is flexed, making it critical for movements that involve turning or rotating the leg away from the midline of the body.

In contrast, the other muscles included in the options have different functions. The adductor brevis is primarily involved in hip adduction and does not contribute to lateral rotation of the leg. The sartorius, while it does assist in hip flexion and abduction, also does not have a significant role in lateral rotation. The gastrocnemius is primarily a plantar flexor of the foot and aids in knee flexion but does not participate in rotating the leg. Therefore, the biceps femoris is the muscle that distinctly fulfills the function of lateral rotation of the leg.