Final answer:
The optimal training activity for stimulating joint mechanoreceptors in a rehabilitation program is proprioception, as it enhances coordination and joint stability through balance and sensory feedback exercises like the Romberg test.
Step-by-step explanation:
The optimal training activity for stimulating joint mechanoreceptors in a rehabilitation program is d. Proprioception. Proprioceptive exercises are designed to improve the awareness of joint position and movement, allowing for better control and stability of the joints. Activities that fall under this type include balance exercises and senses-oriented tasks like the Romberg test, which measures proprioception by having a patient maintain balance with their eyes closed. These exercises utilize the proprioceptive feedback system, which is essential for rehabilitation, as it helps regain the sense of limb orientation without relying on visual cues, thereby enhancing coordination and reducing the likelihood of injury.