Final answer:
The frog has the most gas exchange through its skin due to its permeable skin and the presence of a network of capillaries just below the surface, which is not the case for reptiles like the boa constrictor or humans.
Step-by-step explanation:
The animal among the options given that has the most gas exchange occurring through its skin is the frog. Frogs, as well as other amphibians, have permeable skin that allows for efficient gas exchange with the environment. This skin is moist and contains a network of capillaries just below the surface, which facilitates the dissolution and diffusion of gases. Unlike reptiles such as the boa constrictor, which rely solely on their lungs for respiration due to their scales, frogs use their skin as a significant respiratory organ. This is particularly important for animals like the hellbender, which is a type of salamander, also known for gas exchange through the skin. In contrast, humans have a relatively impermeable skin barrier with a protective layer of keratin, making our skin unsuitable for gas exchange.