What does the term "root" refer to in linguistics?

In linguistics, the term "root" refers to the core part of a word that carries the primary meaning and can often stand alone as a meaningful unit. A root is typically a morpheme, which is the smallest grammatical unit in a language. When considering the choices given, the most accurate interpretation of "root" aligns with it being a bound morpheme that cannot stand alone as a complete word without additional affixes or elements.

For instance, in the word "unhappiness," "happy" serves as the root, while "un-" and "-ness" are affixes that modify its meaning. Therefore, a root can also be part of a family of related words, but the essential characteristic is whether the root can function independently as a word, which in this context adheres to being bound when it needs additional morphemes to create meaning. The term does not apply to standalone phonemes, nor does it refer to a stable but unchangeable form, indicating that roots can evolve through use and formation of new words.