What does it mean if a base word is referred to as a "free" morpheme?

A "free" morpheme refers to a base word that can stand alone as a complete word. This means that it has its own meaning and does not need to be attached to any other morphemes, such as prefixes or suffixes, to convey that meaning. For example, the word "book" is a free morpheme because it can function independently in a sentence, whereas "unhappiness," which includes bound morphemes, cannot stand alone without its components.

The other options describe different characteristics that are not related to the definition of a "free" morpheme. While some morphemes may change meanings or may be required to form a complete word when used with other morphemes, these characteristics do not apply to the fundamental definition of what constitutes a free morpheme. Additionally, the capitalization of a word relates to grammatical conventions rather than the morphemic status of a base word.