Which type of morpheme cannot stand alone?

The correct answer is that a bound morpheme cannot stand alone. Bound morphemes are units of meaning that must be attached to other morphemes to convey meaning. This means they cannot function independently in a sentence or as a standalone word. For example, in the English language, prefixes such as "un-" and suffixes like "-ing" are bound morphemes. They require attachment to a free morpheme—like "happy" in "unhappy" or "walk" in "walking"—to create meaning.

In contrast, free morphemes can stand alone as words that carry meaning by themselves, such as "dog" or "run." Affixes are a subset of bound morphemes and therefore do not stand alone either; they also need to attach to free morphemes. The term "morpheme" itself is broader and includes both free and bound morphemes, but when specifying the type that cannot exist independently, bound morphemes are the focus.