A phoneme is defined as:

A phoneme is defined as a basic unit of speech sounds in a language. It represents the smallest segment of sound that can distinguish one word from another. For example, the words "bat" and "pat" differ by a single phoneme—the initial sounds /b/ and /p/. Understanding phonemes is fundamental in phonics and reading instruction since it helps learners recognize how sounds blend together to form words. This knowledge is essential in developing reading and pronunciation skills.

The other choices relate to different aspects of language but do not accurately define what a phoneme is. A combination of words refers to syntax or phrases, a written symbol pertains to letters or graphemes which represent phonemes but are not phonemes themselves, and a complete sentence involves grammatical structuring rather than the elemental sound unit that phonemes represent.