Which type of morpheme can stand alone as a word?

Free morphemes are types of morphemes that can stand alone as complete words with meaning, such as "cat," "book," or "happy." They do not require other morphemes to convey meaning, which distinguishes them from bound morphemes that cannot exist independently and must be attached to other morphemes to function, like prefixes and suffixes (e.g., un-, -ing).

While complex morphemes refer to those that are formed by combining more than one morpheme, they still rely on free morphemes to make sense. Compound morphemes involve the combination of two or more free morphemes (like "notebook" or "toothbrush") but do not represent a standalone unit of meaning until combined. Therefore, the identification of free morphemes as being capable of standing alone is what makes this category distinct and is the reason for its correctness in this context.