Which of the following is not an example of a prefix?

A prefix is a group of letters added to the beginning of a word to change its meaning. The correct answer identifies "-ing" as not being a prefix because it is a suffix, which is added to the end of a word. The suffix "-ing" usually indicates an action is ongoing, as seen in words like "running" or "jumping."

In contrast, "re-", "un-", and "dis-" are all examples of prefixes. "Re-" can mean to do something again, such as "redo." "Un-" typically indicates negation, as in "unhappy," meaning not happy. "Dis-" also conveys a negative meaning, as in "disagree," which means not agreeing. Thus, the identification of "-ing" is correct, as it emphasizes the differentiation between prefixes, which modify the beginning of words, and suffixes, which modify the end.