Final answer:
The correct answer is option 3) Nervous system. Challenging a client to balance on an unstable surface while barefoot targets the nervous system, which works in conjunction with the musculoskeletal system to maintain balance and posture.
Step-by-step explanation:
When a client is asked to balance on an unstable surface while barefoot, it challenges their nervous system. The nervous system includes both the central and peripheral divisions, which coordinate control of the body through various senses like balance, body position, and touch.
Particularly, the sense of balance, also known as equilibrioception, is crucial for this activity and is primarily managed by the vestibular system located within the inner ear.
However, maintaining balance on an unstable surface also requires the integration of sensory information from the musculoskeletal system, which includes the muscles and joints that provide feedback about body position and movement. Overall, the nervous system's ability to quickly process and respond to these sensory inputs is what allows a person to maintain balance.