Final answer:
The material of choice for a custom mouth protector for this client to wear when playing sports is Thermoplastic.
Step-by-step explanation:
The material of choice for a custom mouth protector for this client to wear when playing sports is Thermoplastic. Thermoplastic materials are widely used for mouth protectors because they can be easily molded and shaped to fit an individual's mouth. This material is also durable and provides good protection against impact.