Final answer:
The fastest conduction velocity in a secondary sensory neuron is achieved by a neuron with a large diameter, myelinated axon, due to the benefits of saltatory conduction and lower resistance within the axon.
Step-by-step explanation:
The secondary sensory neuron with the fastest conduction velocity will be the one with a large diameter and a myelinated axon. Myelination greatly increases the speed of action potential propagation through saltatory conduction, whereby the electrical impulse 'jumps' from one Node of Ranvier to the next, skipping the insulated myelinated segments of the axon. Furthermore, a larger diameter reduces resistance within the axon, allowing for faster transmission of the depolarization signal, analogous to water flowing quickly in a wide river as opposed to a narrow creek.
Therefore, among the options presented, the neuron with a large diameter myelinated axon would be the correct answer, as it combines both features favorable for rapid conduction: myelination and a large cross-sectional area. This configuration enables the neuron to transmit nerve impulses with higher velocity compared to thin or unmyelinated axons.