Concept:The error lies in the use of "either...or" in a negative context; the correct correlative conjunction is "neither...nor".Explanation:The sentence describes that no one could understand the candidate's principles or promises."Either...or" is used when a choice between two possibilities is stated in an affirmative sense.However, the sentence has a negative meaning ("none could clearly understand").In such negative contexts, "neither...nor" must be used to correctly convey that both things are not true.Thus, part (c) should replace "either...or" with "neither...nor".Answer:C