Lethargic means lazy, depressed, whereas vital means full of life. As you can see, these two words are antonyms, meaning that they have completely different meanings. So, in order to finish the analogy, we need to find the antonym of the word trite. Trite means cliche, overused, so its antonym would be original, unique, or something in that vein.
So, the correct answer could be original.
