To create parallel structure in this sentence, which phrase would best replace the words "to prepare the pieces"?
Whenever he built a model ship, he would spend more time researching the ship and photographing the model than he spent to prepare the pieces or assembling the model.
Parallel structure is "using the same pattern of words."
In the sentence, "researching" "photographing", and "assembling" all end in -ing. To create parallel structure, you would want to have an -ing ending to prepare. So the answer is D.