The answer is C. An effector is analogous to the way a ligand (of a stimuli) binds to receptors on the surface of the body and triggers an impulse in the neuron. An effector, on the other hand, occurs internally of the body and when the effector molecule binds to the receptors of the organ that it stimulates or suppresses, a cascade signaling pathway in the cells occurs and the cells/organs responds requisitely.
