Final answer:
Performing calf raises on an incline surface is a better cue for training calf muscles as it increases range of motion and muscle engagement compared to just focusing mentally on the muscles or balance during exercises.
Step-by-step explanation:
When training calf muscles, a cue that works better than thinking about going onto the ball of your foot is C. Performing calf raises on an incline surface. This because an incline increases the range of motion and intensifies the stretch and contraction of the calf muscles during the exercise. Although options A and B can certainly provide a mental focus which can contribute to muscle engagement and growth, option C provides a physical alteration to the exercise that directly targets the muscles being worked. Balancing on one foot during calf exercises can also increase the intensity of the workout but does not directly influence the muscle contraction in the same way as an incline does.