Phonemes are best described as:

Phonemes are defined as the smallest units of sound in a language that can distinguish meaning. Each phoneme represents a distinct sound that can change the meaning of a word if altered. For instance, the difference in sound between the words "bat" and "pat" is the initial phoneme; changing this sound alters the meaning entirely.

This understanding of phonemes is crucial in linguistics and language development as they form the basic building blocks of spoken language. Recognizing and producing phonemes accurately is essential for effective communication, reading, and writing skills. Other terms in the options, such as units of meaning, types of words, or forms of sentences, refer to different linguistic concepts that do not accurately define what phonemes are. Instead, they pertain to morphemes, lexicon, and syntax respectively.