Which example best illustrates the correct use of the genitive case with animals?

The example "The cat's tail is fluffy" effectively illustrates the correct use of the genitive case with animals. In this case, the genitive is formed by adding an apostrophe and an "s" to the noun "cat," indicating possession. This construction shows that the tail belongs to the cat, which is a clear and direct way to express ownership or association.

Using the genitive case in this manner demonstrates a fundamental aspect of English grammar, making it clear that the tail in question is specifically that of the cat, thus avoiding ambiguity. Other options either do not employ the genitive case or do not convey possession, making them unsuitable for illustrating this grammatical concept.