Final answer:
Large-diameter fibers reduce the resistance to the flow of ions, which allows the Na+-based depolarization to spread more quickly down the axon, thus increasing the speed of action potential conduction.
Step-by-step explanation:
The question asks how increased fiber diameter improves the speed of action potential conduction in axons. The correct reason among the given options is that large-diameter fibers reduce the resistance to ion flow. Larger axons offer less resistance internally for the flow of ions, in the same way that water flows more easily through a wide pipe than a narrow one. This reduced resistance allows the Na+-based depolarization to spread more quickly and efficiently down the axon. It's important to note that while myelination plays a significant role in the increase of conduction speed via saltatory conduction, increasing the diameter of the axon also contributes to this rate without solely relying on myelin sheath insulation.