Answer:
Imagery.
Step-by-step explanation:
Figurative language is the use of certain ways of talking about things or events in a text. These figurative elements allow a writer to give certain aspects of describing things.
The given lines from Roald Dahl's "James and The Giant Peach" shows Earthworm describing to the Centipede how they will die. Earthworm's use of words such as "get thinner and thinner and thirstier and thirstier . . .a slow and grisly death from starvation. . . I am slowly shriveling up. . ." provides a descriptive image of how they will look.
Thus, the correct answer is imagery.