What type of morpheme can stand alone as a word?

The correct choice is free morpheme because a free morpheme is defined as a unit of meaning that can stand alone as a word. It doesn't require any additional morphemes to convey meaning. For example, words like "book," "run," and "happy" are all free morphemes since they can function independently in a sentence.

In contrast, bound morphemes cannot stand alone and must attach to other morphemes to provide meaning, as seen with prefixes and suffixes like "un-" in "unhappy" or "-ed" in "talked." Derivational morphemes, which can be bound or free, affect the meaning or part of speech of a base word (for instance, "happy" can become "happiness" when adding a derivational suffix). Functional morphemes, such as prepositions or conjunctions, also do not necessarily convey independent meaning in isolation like free morphemes do.

Thus, it is the free morpheme that specifically meets the criterion of standing alone as a complete word.