Answer:
Root words are parts of words which cannot be divided any further, but which have some kind of meaning on their own. Here, I believe that root words are cred, graph, and vis.
Cred (as found in credible, credibility), means something believable; graph (as in graphic) means to write; vis (as in visible) means to see. Anti, con, and post are all prefixes, which means they are added to words and cannot be root words thus.