Which of the following is NOT a diphthong?

A diphthong is a complex vowel sound that begins with one vowel sound and glides into another within the same syllable. They often represent two distinct vocalic sounds.

The choice identified as 'C' represents "ah," which is a monophthong. A monophthong comprises a single vowel sound that does not change quality during its duration. In contrast, the options that involve "oi," "ou," and "oy" contain two vowel sounds that are articulated smoothly within a single syllable, fitting the definition of diphthongs.

The understanding of this distinction is crucial for identifying various vowel sounds in language development, especially in the context of teaching reading and phonetics, since diphthongs often present unique challenges for learners. Thus, recognizing "ah" as not fitting the criteria for diphthongs aids in grasping how vowel sounds are categorized in linguistic studies.