Final answer:
The neurotransmitter released from sympathetic preganglionic fibers is acetylcholine (ACh).
Step-by-step explanation:
The neurotransmitter released from the sympathetic preganglionic fibers is acetylcholine (ACh). All ganglionic neurons of the autonomic system, in both sympathetic and parasympathetic ganglia, are activated by ACh released from preganglionic fibers. The ACh receptors on these neurons are of the nicotinic type.
When ACh binds to the receptors, a channel opens to allow positive ions to cross the cell membrane, resulting in depolarization of the ganglia.