A diphthong is characterized by:

A diphthong is characterized by a glide from one vowel sound to another within the same syllable. This means that when pronounced, a diphthong begins with one vowel sound and smoothly transitions into another, giving it a unique quality that is different from a single vowel sound. For example, the vowel sound in the word "coin" starts with the vowel sound of "o" and glides into the vowel sound of "i," creating a single, complex sound that blends the two.

While other options describe different phonetic characteristics, they do not capture the essence of what a diphthong is. Simple vowels refer to individual vowel sounds without the glide, consonants do not relate to the definition of a diphthong since they are not vowel sounds at all, and a single vowel sound does not involve the combination of two sounds that defines a diphthong. Thus, the correct choice accurately reflects the linguistic feature that distinguishes diphthongs.