Final answer:
Bright light would most likely cause a person to squint as it initiates a protective reflex reducing the amount of light entering the eye through sensory and motor pathways involving the optic and facial nerves.
Step-by-step explanation:
The stimulus that would most likely cause a person to squint is bright light. Squinting is a reflexive action that helps to protect the eyes from intense light and reduce the amount of light entering the eye. The sensory component of this response is transmitted through the optic nerve, while the motor response is mediated by the facial nerve that activates the orbicularis oculi muscle to decrease the eyelid opening.