Which of the following muscles is primarily responsible for facial expressions?

The buccinator muscle is primarily responsible for facial expressions because it is one of the key muscles of facial expression. Located in the cheek, the buccinator helps to compress the cheeks against the teeth, playing a crucial role in actions such as blowing, sucking, and smiling. This muscle allows for various facial movements by facilitating the movement of the lips and cheeks, which are essential for non-verbal communication and emotional expression.

In contrast, the other muscles listed—rectus femoris, biceps femoris, and semitendinosus—are primarily involved in movements related to the limbs and not the face. The rectus femoris is a muscle of the quadriceps group that assists with knee extension and hip flexion. The biceps femoris and semitendinosus are muscles found in the posterior compartment of the thigh and are involved in knee flexion and hip extension. These functions do not relate to facial expression, which further solidifies the buccinator's unique role in the mechanics of facial movement.