Which smallest unit of sound can include a blend of voice sounds?

The smallest unit of sound that can include a blend of voice sounds is a phoneme. A phoneme is the individual sound segment that makes up words. It represents a single sound in a language, and when these sounds combine, they create different words and meanings.

In discussing the options, graphemes refer to the letters or groups of letters that represent these sounds in written language, and they do not represent auditory units. Diagraphs are combinations of two letters that produce one sound, such as "ch" or "sh," but they do not encompass blends of voice sounds. Diphthongs, on the other hand, are complex vowel sounds formed by the combination of two vowel sounds within the same syllable, such as in the words "coin" or "loud." While diphthongs involve a blend of sounds, they specifically relate to vowel sounds rather than the broader concept of a phoneme, which can apply to both vowel and consonant blends.

Thus, acknowledging the role of phonemes in constructing the basic sound elements of speech, 'B' is the most accurate answer.