Step 1: Understanding the Concept:
The question asks for the common characteristic among crocodile, whale, and hippopotamus.
Step 2: Detailed Explanation:
Crocodile lives in water and on land, whale is exclusively aquatic, and hippopotamus spends most of its time in water. All are aquatic or semi-aquatic animals.
Step 3: Final Answer:
Therefore, the correct option is (D) They are water animals.