Final answer:
The correct option is 1). The Single-leg balance is the best balance exercise for a client in Phase 1 of the OPT model as it focuses on stability and proper movement patterns. More advanced balance exercises, like Bosu ball squats and deadlifts, are typically introduced in later phases.
Step-by-step explanation:
Among the listed exercises, a Single-leg balance is the most appropriate balance exercise for a client training in Phase 1 of the OPT model (Optimum Performance Training model). This foundational phase emphasizes on improving muscular endurance, and stability, and establishing proper movement patterns. The single-leg balance exercise supports these goals by challenging an individual's ability to maintain postural equilibrium, requiring the body to engage multiple stabilizer muscles to remain balanced, and doing so in a controlled and deliberate manner that reinforces correct skeletal alignment and muscle firing patterns.
Exercises like Bosu ball squats and deadlifts are more advanced and would be introduced in later phases of the OPT model as they require a greater level of stability and neuromuscular control. Calf raises enhance muscular endurance and strength in the lower legs, which can indirectly support balance, but they are typically not classified as a primary balance exercise.