Answer: C. sternocleidomastoid muscle
Step-by-step explanation:
The sternocleidomastoid muscle is the most superficial cervical muscle. The primary function of the muscle is the rotation of the head and flexion of the neck.
The brachial plexus does not cause any harm to the sternocleidomastoid muscle because it is innervated by the cranial nerve (accessory) and branches of the spinal nerves C2 and C3 (cervical).