Final answer:
All preganglionic autonomic fibers release acetylcholine at their synaptic terminals, and the effects are generally excitatory due to the activation of nicotinic receptors on ganglionic neurons.
Step-by-step explanation:
All preganglionic autonomic fibers release acetylcholine (ACh) at their synaptic terminals, and while the effects are generally excitatory, the nature of the effect - excitatory or inhibitory - can depend on the type of receptor to which ACh binds. In the autonomic nervous system, both sympathetic and parasympathetic preganglionic fibers release ACh. This neurotransmitter then binds to nicotinic receptors on ganglionic neurons, which are ligand-gated ion channels and cause depolarization.
Therefore, the correct answer is: All preganglionic autonomic fibers release acetylcholine at their synaptic terminals, and the effects are always excitatory, option c).