Which of the following describes phonemes?

Phonemes are the smallest units of sound in a language that can distinguish one word from another. They are not the meanings of words, which is the role of vocabulary (represented in choice A). While letters are related to phonemes, they are not the same; letters are visual representations used in written language (as mentioned in choice B). Phonemes focus specifically on the auditory aspect, representing sounds that can be combined to form words. Therefore, option C accurately describes phonemes as the sounds that letters represent, making it the correct answer in this context.