During the hibernation state, frogs respires through their skin. Option 1, which states that frogs only respire through their heart, is incorrect because the heart is responsible for pumping blood and does not play a direct role in respiration. Option 2, which states that frogs only respire through their lungs, is incorrect because frogs tend to reduce or even stop using their lungs during hibernation as their metabolic rate significantly decreases. Option 4, which suggests that frogs respire through both their lungs and skin, is also incorrect because frogs primarily rely on cutaneous respiration (respiration through the skin) during hibernation. Therefore, option 3, which states that frogs only respire through their skin, is the correct answer.