Answer:
thought about the book.
Step-by-step explanation:
A sentence with a parallel structure is one that uses words, phrases or clauses that are grammatically similar or identical in structure, sound, meaning, or meter, in order to show that the ideas are equally important and give a natural flow to the reading. From all options, the one that would break the parallel structure of the sentence stem is “thought about the book” because the verb “thought” does not repeat the verb form ending in “-ing,” which is present in “sitting” and “reading.”