Explanation:

A. set up: 'Set up' means to establish or arrange something, or to prepare equipment for use. It does not fit the context of expressing views.\nB. set in: 'Set in' means to begin and seem likely to continue (especially of something unpleasant), or to insert or fix something. It does not fit the context of expressing views.\nC. set forth: 'Set forth' means to present, state, or explain (an idea or opinion). This phrasal verb perfectly fits the context of a political party expressing its views at a public rally.\nD. set about: 'Set about' means to begin or deal with something. While it implies starting an action, it is less precise than 'set forth' for the specific act of presenting views.