Final answer:
Balancing on an unstable surface while barefoot challenges the nervous system's ability to maintain balance and control body position. The nervous system must adjust and provide feedback to keep the body stable.
Step-by-step explanation:
The system that would be challenged when asking a client to balance on an unstable surface while barefoot is the nervous system. The nervous system coordinates control of the body using the senses of balance, body position, and touch on the soles of the feet. When balancing on an unstable surface, the nervous system must quickly adjust and provide feedback to maintain balance.