Final answer:
Thick, myelinated axons are large in diameter, heavily myelinated, and convey impulses faster than thin, unmyelinated axons, which are smaller and unmyelinated.
Step-by-step explanation:
The fibers that are large in diameter, heavily myelinated, and convey impulses comparatively faster are thick, myelinated axons. These axons are wrapped in a layer of myelin, which acts as an insulator and facilitates the rapid transmission of nerve impulses through a mechanism called saltatory conduction. This process allows the action potential to 'jump' from one node of Ranvier to the next, thus speeding up the propagation of the nerve impulse. On the other hand, fibers that are smaller and unmyelinated transmit impulses more slowly due to continuous conduction, where the action potential must travel along the entire length of the axon without the benefit of myelin. Therefore, the complete statement would be: "Thick, myelinated axons are large in diameter, heavily myelinated, and convey impulses comparatively faster than thin, unmyelinated axons, which are smaller and unmyelinated."