What is the term for animals that live both in water and on land?

The correct term for animals that live both in water and on land is "amphibians." Amphibians are a class of animals that typically go through a life cycle involving both aquatic and terrestrial stages. This group includes frogs, toads, newts, and salamanders.

Amphibians generally start their life as eggs in water, developing into larvae that are fully aquatic (like tadpoles) before metamorphosing into adults that can thrive in both environments. This dual habitat is essential for their survival, as their moist skin requires them to keep hydrated, which they often achieve by being near water sources.

The other options describe different classes of animals with distinct habitats: mammals primarily live on land, reptiles are mainly terrestrial (though some can be found in water), and fish are strictly aquatic, not having the capability to live on land. Therefore, the adaptability of amphibians to both environments is what defines them uniquely among animal groups.