The correct option is D Skin
Frogs have developed three methods of respiration - buccal, cutaneous, and pulmonary. When submerged in water, they use only their skin for breathing. Since the skin of frog is thin and moist, it allows easy exchange of gases between the blood capillaries (that supply blood to the skin) and the surrounding water. Even on land, the frog's skin remains moist due to the mucus glands in the skin, which enables the animal to continue breathing through its skin. The buccal cavity and lungs are used as additional respiratory structures when the frog is on land.