Which of the following is an example of a prefix?

A prefix is a group of letters added to the beginning of a word to modify its meaning. "Un-" is an example of a prefix because it is placed at the front of a root word to create a new meaning; for instance, adding it to "happy" forms "unhappy," indicating the opposite of happy.

In contrast, the other options are all examples of suffixes, which are added to the end of a word. "-ed" denotes the past tense in verbs, "-ing" indicates an ongoing action or state, and "-ness" is used to form nouns that express a state or quality, such as "kindness." These forms do not appear at the beginning of words, which is a defining characteristic of prefixes.