Final answer:
Ib GTO afferents exert excitatory effects on antagonistic muscles.
Step-by-step explanation:
Ib GTO (Golgi tendon organ) afferents exert excitatory effects on antagonistic muscles. Antagonistic muscles are the muscles that oppose or resist the movements of agonist muscles. In the context of muscle movement, agonist muscles are the prime movers while antagonists work to oppose their movements. Synergists assist the agonists, and fixators stabilize a muscle's origin. Therefore, the correct option is b. Antagonistic muscles.