Final answer:
To assess a client's head and neck hyperextension, a nurse would instruct the client to 'Lower your chin toward your chest and then tilt it backward.' This allows the evaluation of the neck muscles, particularly the sternocleidomastoid, which is involved in the rotation and flexion of the head.
Step-by-step explanation:
To assess the hyperextension of a client's head and neck, the correct instruction that the nurse should provide is option C: 'Lower your chin toward your chest and then tilt it backward.' This action will allow the health professional to observe the neck muscles as they control the motion of the head, specifically in hyperextension, which involves moving the head back beyond its normal range of motion. Notably, the sternocleidomastoid muscle plays a significant role in flexing and rotating the head, contributing to the range of motion that can be evaluated during a physical examination.