Hi,
From what i understand, the author says that there were other influences on the acculturalization of the Cherokees, namely those from the "White resident traders and neighbors", which "obscured the relative impact of the missionaries" (line 56). The author found this to be a weakness in McLoughlin's argument.
On line 54, the author says that, "McLoughlin unfortunately overlooks earlier sources of influence,..." thereby implying that the acculturization in the 1820s is probably a just a continuation of an "already-existing process of acculturization".
On the other hand, choice C does not address those other factors presented by the author in the last paragraph. If the wording was "could have been the result of earlier contacts with Western resident traders and neighbors...." then it would've been a better answer.
Hope it helped.