What defines a nonphonemic diphthong?

The correct choice highlights that a nonphonemic diphthong is a diphthong used in stressed syllables but can also be represented by a single vowel. This means that, although the diphthong may be pronounced as two vowel sounds in that specific context, it does not change the meaning of the word if it is replaced by a single vowel. In phonetics, distinguishing between phonemic and nonphonemic diphthongs is essential because phonemic diphthongs create a contrast in meaning, while nonphonemic diphthongs do not.

In this context, the key takeaway is that the presence of a nonphonemic diphthong allows for flexibility in pronunciation without altering semantic distinction, particularly within stressed syllables. This reflects the dynamic nature of spoken language and how pronunciation can vary based on stress and syllabic emphasis.