Which term best describes a blend of vowels producing one sound in a syllable?

The term that best describes a blend of vowels producing one sound in a syllable is "diphthong." A diphthong occurs when two vowel sounds are combined within the same syllable, resulting in a smooth glide from one vowel sound to another, but essentially functioning as a single speech sound.

For example, in the word "coin," the vowel sounds in "o" and "i" blend together to create a new sound that is perceived as a single unit rather than two distinct sounds. This characteristic allows diphthongs to be a crucial element in phonetics and phonology.

The other terms do not represent this concept accurately. Connotation refers to the implied meanings or associations of a word beyond its literal definition. Epenthesis describes the addition of one or more sounds to a word, often for ease of pronunciation. Assimilation involves a phonetic process where a sound changes to become more like a neighboring sound, which is distinct from the blending of vowels into a single sound.