Which morpheme cannot stand alone and is often attached to others?

The correct choice is the bound morpheme, which cannot stand alone as a word and must be attached to other morphemes to convey meaning. Bound morphemes include prefixes (like "un-" in "unhappy") and suffixes (like "-ed" in "walked"). These morphemes modify the meaning of the free morphemes they attach to but do not have meaning on their own.

In contrast, free morphemes are words that can stand alone, like "cat" or "run." Inflectional morphemes are a type of bound morpheme that specifically serve grammatical purposes, such as indicating tense or plurality. Derivational morphemes, while they may also be bound, involve a process that creates new words or alters the meaning significantly when attached to free morphemes. However, the key feature of bound morphemes is their inability to function independently, which defines their role in language structure.