Final answer:
The fastest propagation of an action potential occurs in a thick, myelinated axon due to less electrical resistance and saltatory conduction.
Step-by-step explanation:
The type of fiber where an action potential will propagate most quickly is a thick, myelinated axon.
The speed of an action potential's propagation depends on several factors related to the structure of the nerve fiber. Actions potentials move faster in axons that are thicker because they have a greater diameter, which reduces electrical resistance. Another critical factor is myelination; axons that are myelinated have a sheath around them that allows the action potential to 'jump' from one Node of Ranvier to the next in a process called saltatory conduction. This process greatly increases the speed of propagation compared to an action potential moving along an unmyelinated axon. Thus, among the given options, option d, a thick, myelinated axon, will have the fastest propagation of an action potential due to the combined effects of having a larger diameter (less resistance) and the presence of myelin (faster signal transmission). Additionally, the metabolic properties of muscle fibers, such as how fast they hydrolyze ATP, indicate how quickly they contract, but this relates to muscle action rather than neural transmission.