The correct option is A Ammonia
Animals that excrete excess nitrogen in the form of ammonia (the end product of protein metabolism) are called ammonotelic. Ammonia diffuses through the cell membrane extremely fast, because of its high water solubility and small molecular size. Therefore, prompt excretion of ammonia occurs in aquatic animals (aquatic invertebrates, fishes, larvae, permanently aquatic amphibia). The route of ammonia diffusion in hydra is through skin.