Hey gmat1220 - you know, I don't want to guarantee that we NEED that -ing modifier, but:
-As Baten80 mentioned, the singular modifier "a plan" doesn't really work with "acted on plans (plural)"
-The -ing modifier is definitely correct in this case, as it modifies the entire preceding clause and isn't limited just to the word or phrase immediately adjacent to the comma. Because of that, I mentioned before that I'd tend toward that modifier if I were at all unclear about the validity of the "resumptive modifier" you mentioned and whether it would apply.
So it's not that the situation can only call for exactly one type of modifier...I just know that the modifier in C is clearly allowable, and that D introduces a few doubts (at best) and contains a fatal verb tense flaw. And since one fatal flaw dooms an answer, and I found one in D and can't find any in C, there are a few reasons to make that choice.
I hope that helps - I'd be careful about limiting yourself to exactly one type of phrasing that you think you NEED...Sentence Correction is much more about eliminating flaws.
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