Which of the following describes diphthongs?

The description of diphthongs as a combination of two vowel sounds in one syllable accurately captures their essence. Diphthongs are distinctive because they involve a smooth, gliding transition from one vowel sound to another within the same syllable. For example, the word "coin" includes the diphthong sound that starts with an 'o' and glides toward an 'i'.

This definition is crucial as it highlights that diphthongs are not merely two separate vowel sounds pronounced one after the other; rather, they blend together seamlessly to form a single auditory unit. Recognizing diphthongs helps learners understand more complex vowel sounds in English, as they contribute to the richness and variety of pronunciation in the language. The other options do not capture this process accurately, focusing instead on aspects of vowels or consonants that do not pertain to the specific nature of diphthongs.