Final answer:
The ranking is lizard, human, frog for increasing gas exchange through the skin, with lizards having the least skin gas exchange and frogs the most.
Step-by-step explanation:
The order of increasing gas exchange through skin as a percent of total gas exchange for the organisms listed would be lizard, human, frog. Lizards have scales that prevent significant gas exchange through the skin and instead rely primarily on their lungs for respiration.
Humans have skin with sweat glands and a high degree of vascularization, but their primary mode of gas exchange is through the lungs which contain numerous alveoli for this purpose.
Frogs, on the other hand, have a high level of gas exchange through their skin. Their integument must be kept moist for this respiratory process to be efficient, supported by a network of capillaries just below the skin for gas exchange.