Which definition accurately describes a morpheme?

A morpheme is indeed defined as a meaningful linguistic unit. It serves as the smallest segment of language that carries meaning. This could include a single word, such as "cat," which stands alone as a complete idea, or parts of words, like "un-" in "unhappy" or "-ed" in "talked," which modify the meaning of a base word. Understanding morphemes is essential for grasping how words are structured and how they convey different meanings through composition and alteration.

The other definitions do not accurately depict a morpheme. A structural rule for creating sentences relates more to grammar and syntax rather than the smallest meaningful unit of language. A sound-based unit of language pertains to phonemes, which are the smallest units of sound that distinguish one word from another, rather than units with specific meanings. While the smallest unit of speech may seem relevant, it doesn't encapsulate the entire concept of meaning that a morpheme includes. Thus, the most accurate description is that a morpheme is a meaningful linguistic unit.