Which of the following is an example of a single-reed woodwind instrument?

The clarinet is correctly identified as a single-reed woodwind instrument because it produces sound through the vibration of a single reed attached to its mouthpiece. This is a defining characteristic of single-reed instruments, which also include others like the saxophone.

In contrast, the flute is a non-reed instrument that produces sound through the division of airflow across an opening. The harmonica, while featuring reeds, is categorized as a free reed instrument, where the sound is generated by air passing over multiple reeds, not through a mouthpiece like the clarinet. The oboe, on the other hand, is a double-reed woodwind instrument, using two reeds that vibrate against each other to create sound, which distinguishes it from clarinets. Thus, the clarinet is the correct choice as an example of a single-reed woodwind instrument.