What unique feature do axolotl gills possess?

The unique feature of axolotl gills that makes option C the correct answer is their feathery structure, which is specifically adapted to increase the surface area available for gas exchange. This adaptation is crucial for the axolotl's respiratory efficiency, allowing it to absorb oxygen from the water more effectively. The extensive branching of the gill filaments provides a larger surface area relative to their volume, which is vital for meeting the oxygen demands of the axolotl, especially in its aquatic environment.

The gills of axolotls are external structures that protrude from the sides of their heads, setting them apart from many other amphibians that have internal gills covered by an operculum. This external positioning and the feathery design are specialized adaptations that play a significant role in their survival in a juvenile, aquatic form.