Final answer:
Tadpoles use gills to breathe in water. These gills allow them to extract oxygen directly from the water. As they mature, amphibians develop lungs and can also breathe through their skin.
Step-by-step explanation:
Young tadpoles breathe like fish, using gills to absorb oxygen directly from the water, which is the correct structure that enables them to breathe in the water (Option A. Gills). As amphibians, tadpoles undergo metamorphosis and develop into froglets with lungs for terrestrial breathing. Amphibians have the unique ability to also perform cutaneous respiration, where they use their moist, permeable skin to absorb oxygen.