Final answer:
The Khaen is the mouth organ associated with northeastern Thai folk music and phleng luk thung style, classified as an aerophone in ethnomusicology.
Step-by-step explanation:
The mouth organ common to northeastern Thai folk music and frequently used in the popular style known as phleng luk thung is the Khaen (A). The Khaen is an aerophone, which is a classification of musical instruments in ethnomusicology where sound is produced primarily by causing a body of air to vibrate, without the use of strings or membranes.
Aerophones are one of the four kinds defined in ethnomusicology, the others being chordophones (stringed instruments), membranophones (instruments with a membrane, such as a drum), and idiophones (self-sounding instruments).