Concept:Group the animals by their habitat and ability to survive on land as well as in water.
Explanation:A tortoise, though often land-dwelling, can survive in water and is classified as a reptile that does not depend entirely on water.
A crab lives near water but can easily move and survive on land, so it is not strictly aquatic.
A frog is an amphibian, meaning it lives both in water and on land during its life cycle.
Fish, however, is the only animal in the list that is completely aquatic and cannot live outside water.
It breathes through gills and has no limbs to survive on land.
Therefore, the odd one out is the one that differs in this habitat feature.
Answer:Option D – Fish.