Which adductor muscle is involved in hip adduction?

The gracilis is the correct answer because it is one of the primary muscles responsible for hip adduction. Located in the inner thigh, the gracilis assists in bringing the thigh closer to the midline of the body. It plays a significant role not only in adduction but also in flexion and medial rotation of the hip joint.

The other muscles listed do not primarily function as adductors of the hip. The semitendinosus, while a part of the hamstring group, is primarily involved in flexing the knee and extending the hip rather than adducting it. The sartorius is known for its role in flexing, abducting, and laterally rotating the hip, and does not contribute to adduction. The rectus femoris, primarily a knee extensor and hip flexor, also does not have a role in hip adduction. Thus, the gracilis stands out as the muscle specifically engaged in the action of hip adduction.