Which example illustrates a true diphthong?

A true diphthong is a complex vowel sound that begins with one vowel sound and glides into another within the same syllable. The choice that illustrates a true diphthong is the sound represented by the letter /o/ as in 'go' with a line over it, which indicates that it is a diphthong. This sound starts at a certain vowel position and moves toward another, smoothly changing during its articulation, making it distinct from monophthongs, which maintain a single vowel quality throughout.

In contrast, the other examples represent monophthongs. The sound /e/ as in 'bed' is a pure vowel that does not involve a glide; it remains static. Similarly, /a/ as in 'cat' is a straightforward vowel sound with no change in quality. Finally, /ɛ/ as in 'set' also represents a stable vowel segment without the gliding characteristic that defines a diphthong. Thus, the choice of /o/ as in 'go' is correct as it demonstrates the fluid movement between two vowel sounds, embodying the essence of a true diphthong.