Parallel structure is using the same pattern of words or using the same grammatical form. In this passage, "researching," "photographing," and "assembling" are all similar because of the -ing. So to make the phrase "to prepare the pieces" flow nicely, the best way to rewrite it is D, preparing the pieces. The -ing in "preparing" is similar to the pattern of the other words.
This <span>sentence in the expert supports the claim that the Americans colonies could thrive independently from Britain. The expert is George Washington</span>
