What is a diphthong?

A diphthong is accurately defined as a complex vowel sound that begins with one vowel sound and glides into another within the same syllable. This explains why the answer is correct, as diphthongs involve the articulation of two distinct vowel sounds in a smooth, continuous motion, creating a unique sound.

This phenomenon can be observed in words like "coin" and "out," where the transition between vowel sounds occurs seamlessly. Recognizing diphthongs is crucial for understanding phonetics and pronunciation, as they represent a significant aspect of how sounds combine in spoken language.

The other choices do not capture the essence of diphthongs. The first option describes a single vowel sound, which does not account for the gliding component that characterizes diphthongs. The third option refers to consonant blends, which involve multiple consonants rather than vowels. The fourth option about syllables in compound words does not relate to the nature of vowel sounds specifically. Each of these alternatives diverts from the core characteristics that define a diphthong.