Which of the following is a diphthong?

A diphthong is a complex vowel sound that begins with one vowel sound and glides into another within the same syllable. In English phonetics, the combination of sounds represented by "ow" is an example of this, as it starts with an "o" sound and transitions into a "u" sound (as in "cow" or "how").

The other options represent different phonetic elements. The "ea" combination can create a single vowel sound (as in "bread" or "bead") rather than gliding between two vowel sounds. "Th" and "st" both represent consonant sounds rather than vowel sounds; "th" can represent voiced or voiceless dental fricatives (as in "this" or "thin"), and "st" represents an initial consonant cluster.

Understanding diphthongs is essential for language learners, particularly in English pronunciation, as they can significantly affect the meaning and clarity of words.