In a cardboard representation of nucleotide subunits, Watson and Crick

Hey yuktipoddar,

This is a really good example of prioritizing your decision points. This doesn't have to be a case of present versus past participles. It can be a case of tenses and complete versus incomplete sentences.

In terms of tenses, you know a few things: Watson and Crick modeled DNA - that means they did this in the past, since you have that -ed ending on modeled. That means that the scientists who came before needed to do their models before Watson and Crick did theirs. And since doing a model is something that ends, you know that process was done by the time Watson and Crick started. That means that you need "had failed" to correctly deal with that timeline. That eliminates everything except A and E.

Take a look at what happens if you have -ed in there (as it is in E): "In a cardboard representation of nucleotide subunits, Watson and Crick, elaborated on other scientists’ theories which had failed to explain the nucleotide structures and pairings in their entirety, accurately modeled the double-helix DNA." That first part (up to "entirety") would be a complete sentence if that comma wasn't there. You need that "-ing" ending in there to make it clear that you aren't trying to make a compound sentence (because if you were trying to make a compound sentence you'd need an "and" after that comma or you'd need to change "modeled" to "modeling")