Which aspect of language deals with the smallest units of sound?

The aspect of language that deals with the smallest units of sound is phonemes. Phonemes are the individual sounds that make up words in a language. For example, in the English word "cat," there are three phonemes: /k/, /æ/, and /t/. These sounds are the building blocks of speech, and changing a single phoneme can change the meaning of a word, such as changing the /k/ in "cat" to /h/ to form "hat."

Morphemes, on the other hand, are the smallest units of meaning in a language. They can be whole words or parts of words (like prefixes or suffixes), but they pertain to meaning rather than sound. Sentences and clauses represent larger linguistic structures that contain both phonemes and morphemes but do not focus on the smallest units of sound. Therefore, phonemes are specifically defined as the smallest distinct sound units in language, making them the correct answer to this question.