Which of the following is an example of a diphthong?

A diphthong is a complex vowel sound that begins with one vowel sound and glides into another within the same syllable. The correct answer illustrates this concept perfectly.

The "ou" in "sound" is a classic example of a diphthong because it combines two vowel sounds: the "o" sound and the "u" sound, creating a unique vowel sound that shifts from one to the other while being pronounced. This glide from one sound to another within a single syllable is what characterizes diphthongs.

In contrast, the other examples represent different phonetic phenomena. The "ee" in "tree" is a single vowel sound and does not glide into another vowel; it is a long vowel. The "ch" in "chair" and the "sh" in "ship" are both consonant blends. These sounds do not contain the vowel glide characteristic of diphthongs, making them distinct from the given correct choice.