D. "this was an uneasy secret..."
Only things ending in -ing would ensure parallelism in this sentence
there for the answer is "thought about the book"
To prevail is to ‘gain the desired effect’. It’s kind of like achieving a goal, or overcoming the odds.
Like in a book or a movie, when the hero prevails, they usually defeat the villain, resolve a conflict, or achieve whatever goal(s) they had.
Hope this helps.
