Final answer:
Thurber uses hyperbole as an imaginative comparison.
Step-by-step explanation:
In the excerpt from "The Day the Dam Broke," Thurber uses hyperbole as an imaginative comparison. Hyperbole is a figure of speech that involves exaggerated statements or claims not meant to be taken literally. In this case, Thurber uses the comparison "as safe as kittens under a cookstove" to emphasize the feeling of safety despite the presence of despair. By comparing the characters' safety to that of kittens under a cookstove, Thurber effectively conveys the extent of their perceived safety.