What describes a morpheme that can only appear as part of a larger word?

A bound morpheme is correctly identified as a morpheme that cannot stand alone and must be attached to a free morpheme to convey meaning. Examples of bound morphemes include prefixes like "un-" in "unhappy" or suffixes like "-ed" in "walked." These morphemes add specific grammatical or semantic information to the words they are attached to but are unable to function independently as words themselves.

This understanding is crucial in the study of morphology, as it helps in distinguishing between different types of morphemes and their roles in word formation. In contrast, free morphemes can stand alone as words (e.g., "cat," "book"), affixes are a subtype of bound morphemes that specifically refer to prefixes or suffixes, and lexical morphemes refer to those that have a specific meaning and can function as content words, such as nouns and verbs.