first, i wouldn't worry much about this question -- it's from an author who is apparently so unfamiliar with the test that he/she doesn't even know the most basic facts about how these questions are formatted. (specifically, the underlines all start with the same words; this NEVER happens on official problems, on which the underline ALWAYS starts at the first difference and ends at the last difference).

(a) is fine.
(c) would be fine if a comma were added before the -ING modifier. we need the -ING modifier to modify the previous clause, a type of modification that calls for a comma.

VivianKerr wrote:I also would choose C, for reasoning similar to Frankenstein. The implication is that he backed the innovation BY saying it would have to be used. Though parallel, "by backing...and saying..." implies they are 2 different things, which in context they are not.

i hear you here, but this isn't necessarily true; the context is not specific enough to allow us to make this determination for sure.
i.e., BOTH contexts are possible:
(a) the person backed the innovation in some other way (i.e., by justifying it in terms of rules/logic), AND THEN said that it would have to be used.
(c) the person backed the innovation by actually saying that it would have to be used.

both meanings are fine.
on balance, then, the original (a) is the best choice -- although the gmat is not going to have an answer choice like (c). i.e., the gmat is not going to contain answer choices that are only wrong in terms of punctuation.