What 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. In this case, the correct choice showcases a combination of vowel sounds that exemplifies this characteristic.

In the word "Mounds," the vowel sound represented by "ou" is a classic example of a diphthong. It starts with the vowel sound /aʊ/ and moves to a slightly different position as it is pronounced, creating a blend that is typical of diphthongs. This creates a fluid sound that is distinct from a simple vowel, which would maintain a single position in the mouth without gliding to another sound.

The other choices do not contain diphthongs. For instance, "Skittles" contains simple vowel sounds rather than a gliding combination, "York" has a single vowel sound, and "Twix" also features a straightforward vowel. Thus, "Mounds" is the clear choice demonstrating the definition of a diphthong.