The question is about understanding the role of 'standard English'. The correct answer is C because the statement suggests that standard English can best convey a variety of messages, implying effectiveness in communication. Option A is incorrect because it suggests a negative effect, which is not implied in the statement. Option B is incorrect because it contradicts the idea of being 'best' for conveyance. Option D is incorrect because it implies exclusivity, which is not consistent with the concept of a standard language that is meant to be widely understood. The key to this question is recognizing the term 'best convey' as indicating effectiveness in communication.