Concept:A diphthong is a vowel sound where the tongue moves from one position to another, creating a sliding or gliding effect within a single syllable.Explanation:Diphthongs start with one vowel sound and glide smoothly into another. For example, "oy" in "boy" and "ou" in "cloud" are diphthongs. Because of this gliding nature, they are also called gliding vowels or vowel glides. This makes "Vowel glides" the correct term. In contrast, pure vowels (monophthongs) have a fixed sound without any glide. Semi-vowels like "w" and "y" are speech sounds that function between vowels and consonants. Weak vowels occur only in unstressed syllables, such as the schwa sound.Answer:Option D: Vowel glides