Final answer:
Conduction velocity is more rapid in a myelinated axon due to saltatory conduction, where the action potential leaps from node to node, and also influenced by the axon's diameter. Therefore, the correct answer is: A) Myelinated.
Step-by-step explanation:
The action potential propagation is more rapid in a myelinated axon than an unmyelinated one. Myelinated axons facilitate saltatory conduction, where the electrical impulse 'jumps' from one node of Ranvier to the next, bypassing the insulated regions of the axon. This increases the speed of conduction compared to continuous conduction in unmyelinated axons, where voltage-gated Na+ channels must open sequentially along the entire length of the axon.
Therefore, the correct answer is: A) Myelinated. Additionally, an axon's diameter affects the speed of conduction because a wider axon has less internal resistance, allowing for faster signal transmission.