Final answer:
The longest type of fiber in the autonomic nervous system is the preganglionic parasympathetic fiber. This is because these fibers travel from the cranial or sacral regions to terminal ganglia near or within the target organs, resulting in longer preganglionic fibers and very short postganglionic fibers.
Step-by-step explanation:
When comparing the types of fibers in the autonomic nervous system, the longest would be the preganglionic parasympathetic fibers. The explanation for this lies in the structure of the autonomic nervous system: while the postganglionic sympathetic fibers are long because they must travel from the ganglion to the target organ, the parasympathetic system has a different layout. The preganglionic parasympathetic fibers originate in the cranial region or the sacral region and travel a considerable distance to reach terminal ganglia, which are located near or within the target effector organs. The postganglionic fibers in the parasympathetic system are short because these terminal ganglia are usually very close to the effector organs. Thus, answering the question, the type of fiber that could be considered the longest is the preganglionic parasympathetic fiber.