The gluteus maximus is primarily responsible for hip extension, which is the correct answer. This muscle plays a critical role in movements that involve straightening the hip, such as standing up from a seated position, climbing stairs, and running. When the hip extends, the gluteus maximus contracts to pull the thigh backward, enabling these powerful movements.
In contrast, knee flexion involves the bending of the knee joint, which is primarily facilitated by muscles in the hamstring group. Hip adduction refers to moving the thigh toward the midline of the body and is mainly performed by muscles located in the inner thigh, such as the adductors. Hip abduction involves moving the thigh away from the midline, which is the role of the gluteus medius and minimus muscles. Therefore, hip extension is the specific action that highlights the primary function of the gluteus maximus.