Answer:
Selena could not decide which was the cleverest: the skeleton bride holding flowers, the skeleton cowboy holding a lariat, or the skeleton poet holding a book.
Explanation:
Parallel structure (also called parallelism) is the repetition of a chosen grammatical form within a sentence.
The given sentence has three nouns (bride, cowboy, poet) used with the same attributive noun skeleton before each noun and the same participle holding after it. That's parallelism.
The other sentences do not have all of these identical patterns, and, therefore, there is no parallelism in those sentences.