In democracy, decisions are made based upon the majority rule, which is a voting rule that decisions are made based the majority of the votes, meaning more than half of the votes. However, majority rule also poses a threat to democracy because, if decisions are always made by the majority, then there is a risk that citizens will be underrepresented, that "representative democracy will not work for all of its citizens"; minorities have the right to be represented as well (Guy-Uriel Charles, "Dissent, Diversity, and Democracy," Duke University). However, political theorists have also understood that the majority rule does not pose a substantial threat to democracy because "majority coalitions are transitory," meaning that who is in the majority and what beliefs are being held will always shift (Charles). So long as we "take turns ruling and being ruled" democracy works (Charles).
Hence, when we speak of shifting majority, we are referring to the fact that neither the majority nor the minority statuses are fixed; they will always change with the times. Therefore, of your multiple choice questions, the concept of shifting majority refers to the fact that (C) "no voter is always in the majority."