The muscles involved in jaw movement are fourfold: lateral pterygoid, masseter, temporalis, and medial pterygoid. The lateral pterygoid opens the jaw while the other three close the jaw. All four are innervated by the trigeminal nerve (V3). One can remember this with the mnemonic “M’s Munch” and “Lateral Lowers.” Yawning would be impaired with a lateral pterygoid injury.
Answer A is incorrect. Closing the mouth is performed by the masseter, temporalis, and medial pterygoid.
Answer B is incorrect. Smiling requires the function of many facial muscles, including the zygomaticus major.
Answer C is incorrect. Swallowing is mediated by the pharynx and the tongue.
Answer D is incorrect. Whistling is mediated by the tongue and facial muscles, such as the buccinator and orbicularis oris.