Concept:Amphibians use their moist skin as an additional respiratory surface for gas exchange.
Explanation:In frogs, the skin is thin, moist, and well supplied with blood capillaries.
This allows oxygen to diffuse through the skin into the bloodstream and carbon dioxide to diffuse out.
Rats (mammals) rely mainly on lungs.
Lizards (reptiles) also breathe with lungs.
Fish respire through gills.
Therefore, among the options, the frog is the vertebrate whose skin functions as a respiratory surface.
Answer:D. frog