Final answer:
The spinal accessory nerve (CN XI) is one of the twelve cranial nerves and is not part of the cervical plexus. Its main function includes the contraction of neck muscles, so the correct answer is B.False.
Step-by-step explanation:
The spinal accessory nerve, also known as CN XI or the eleventh cranial nerve, is responsible for the contraction of neck muscles, including the sternocleidomastoid and trapezius muscles. It is not part of the cervical plexus; instead, it is one of the twelve cranial nerves that are attached to the brain and primarily serve the sensory and motor functions of the head and neck. Therefore, the answer to the question is B.False.