Which of the following best defines an affix?

An affix is best defined as a morpheme added to a base word. This definition captures the essence of what an affix is in linguistics. Affixes can be either prefixes or suffixes, which means they are added to the beginning or end of base words (or root words) to modify their meanings or create new words. For example, adding the prefix "un-" to the word "happy" forms the word "unhappy," thus changing its meaning.

The concept of an affix being a morpheme emphasizes its role as the smallest unit of meaning in a language. This understanding is crucial for distinguishing affixes from standalone words, which do not require additional morphemes to convey meaning, and from phrases that are not necessarily tied to the formation of words.