The correct option is C Cyclostomates
Myelin is considered a defining characteristic of the (Gnathostome) vertebrates only. Lampreys and hagfishes are the only jawless vertebrates to survive today. They both have a round mouth and for this reason, are often referred to as Cyclostomes. They are the most primitive of the vertebrates. The axons of their neurons are unmyelinated (like those of all invertebrates). So, the correct answer is option D.