Final answer:
The incorrect sentence is "The prefix of unanimous is -un." Unanimous is not composed of a prefix and a root word; thus, it does not have a prefix.
Step-by-step explanation:
The sentence that uses a word incorrectly is "The prefix of unanimous is -un." The prefix of the word unanimous is not -un because unanimous does not have a prefix; it's a single morpheme word derived from the Latin unanimitas. The other sentences in the question are using their words correctly.
In English, prefixes are added to the beginning of a word to change their meaning. Common prefixes include un-, such as in uncertain, in-, as in invisible, or re-, as used in rewrite. Words that do not split into smaller meaning units, like unanimous, do not contain prefixes.