Treating every gender exactly the same. paying the same. acting the same
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.
I think the answer may be C or D.
Hope this helps!
Let me know if i'm wrong