The best characteristic that can be used to separate cats from rabbits among the provided options is teeth shape.
Cats are carnivores and have sharp, pointed teeth for tearing meat, while rabbits are herbivores and have flat, broad teeth suited for grinding plant material.
While all mammals have backbones, fur is not a distinguishing feature unique to cats versus rabbits, and whiskers are present in many different animals. Therefore, the most specific option is teeth shape.