Explanation:

- During hibernation, frogs rely on cutaneous respiration, which means they breathe through their skin.

- Option 3: Only skin is the correct answer.

- Frogs have a permeable skin that allows the exchange of gases, which is crucial during hibernation when they are not using their lungs.

- Option 1: Only heart is incorrect. The heart is vital for circulation, but not directly for respiration.

- Option 2: Only lungs is incorrect. The lungs are not primarily used during hibernation as frogs are less active.

- Option 4: Both lungs and skin is incorrect regarding hibernation because lung activity is minimal.

.