The spinal accessory nerve is crucial for which of the following functions?

The spinal accessory nerve, also known as cranial nerve XI, plays a vital role in motor control, particularly in the movement of the head and shoulders. It primarily innervates two key muscles: the sternocleidomastoid and the trapezius. These muscles are essential for the ability to turn the head side to side and to elevate the shoulders. As such, proper functioning of the spinal accessory nerve is critical for these specific movements, making it integral to neck rotation and shoulder elevation.

The other choices do involve different cranial nerves or structures. For example, facial expression is primarily controlled by the facial nerve (cranial nerve VII), swallowing involves multiple cranial nerves including the glossopharyngeal (cranial nerve IX) and vagus (cranial nerve X), and hearing is covered by the vestibulocochlear nerve (cranial nerve VIII). Therefore, the correct function associated with the spinal accessory nerve is clearly linked to the turning of the head and shoulders.