What sound is represented by the symbol aʊ?

The symbol aʊ represents a diphthong, which in phonetics refers to a complex vowel sound that begins with one vowel sound and glides into another within the same syllable. In this case, the aʊ diphthong starts with an open back to near-open, and glides to a near-high back, which is phonetically similar to what we perceive as the "ow" sound as in the word "how" or "cow." This is why the correct answer identifies this sound representation as the "ow" sound.

The other choices are misrepresentations of the vocal qualities associated with the aʊ sound. A short 'a' sound would be represented by a different phonetic symbol, often more in line with the sound found in "cat." The long 'o' sound typically corresponds to a single phoneme that would be denoted differently, rather than a diphthong sound. Lastly, the 'ah' sound corresponds to the short 'a' or 'ɑ' sound, which is distinctly different from the aʊ diphthong, affirming that the best match for the sound represented by aʊ is indeed the "ow" sound.