Which of the following best describes a phoneme?

The answer is well-aligned with the definition of a phoneme. A phoneme is indeed the smallest sound unit in a language that can change the meaning of a word when altered. For instance, changing the phoneme in the word "bat" to "pat" changes the meaning of the word entirely. This concept is essential in phonetics and phonology, as it emphasizes the role of sound in linguistic meaning and communication.

Understanding phonemes is crucial for language acquisition, especially for learners of English, as it helps them differentiate between words that may sound similar but have different meanings. This distinction shows the importance of phonemic awareness in literacy development, as recognizing and manipulating these sounds is foundational to reading and writing.

The other options provided do not accurately capture the essence of what a phoneme is. The smallest unit of meaning refers to a morpheme, a group of letters that form a word signifies a lexical item rather than a sound, and the structure used to form sentences refers to syntax, all of which are separate concepts in linguistics.