A patient has injury to the nerve exiting the stylomastoid foramen entering the middle ear on the right side. Which muscle is going to be affected?

The nerve exiting the STYLOMASTOID FORAMEN is the FACIAL NERVE (CN VII). The facial nerve provides MOTOR innervation to all muscles of FACIAL EXPRESSION, including the BUCCINATOR. If injured at the stylomastoid foramen, the patient would have ipsilateral facial paralysis (Bell's palsy-like) affecting all facial muscles on that side — including buccinator, orbicularis oris, frontalis, etc. Masseter, medial and lateral pterygoid are innervated by the TRIGEMINAL nerve (V3). A is correct.