The correct answer is: vis, cred and graph, are root words.
Anti, con and post, are prefixes.
To determine if a morphema is a root word is not an easy task. Breaking words into parts is quite the best method to discover what the root word is, because the root word will be the morphema left after removing the prefix and/or the suffix, and it shall be the morphema holding the most basic meaning of a word.
Words not always have affixes (prefixes, suffixes), and there are some words which may have two root words combined. Some of them can be standalone words (like phobia), while others cannot stand alone (like vis). In any case it should be underlined that the root word is the core word, without which the particular word would not have any meaning.