Ok, thanks rmk that makes a lot of sense when I think of it as a prinicipal question. However, how did you know to think of it as that.... isn't a fill in the blank questions supposed to be must be true questions? if so i don't see how either C or D must be true. For C we can't say anything about the most talented writers must be true since the argument doesn't discuss them like you said and for D a writing style shouldn't be more complex for Poor writers, but the argument never says that is the case for all writers and thus couldn't D be false.
Basically, I could see D only if the question read which one of the following principles would most justify the question