Final answer:
Unilateral contraction of the sternocleidomastoid muscle results in ipsilateral cervical lateral flexion (tilting toward the same side) and contralateral rotation (turning to face the opposite side).
Step-by-step explanation:
When the sternocleidomastoid muscle contracts unilaterally, it produces two distinct motions in the head and neck. The first motion is ipsilateral cervical lateral flexion, meaning the head tilts toward the shoulder on the same side as the contracting muscle. The second motion is contralateral rotation, where the head turns to face the opposite side. Therefore, the correct answer to the question 'What motion is produced by unilateral contraction of the sternocleidomastoid muscle?' is C. Ipsilateral cervical lateral flexion and contralateral rotation.