What defines idiophones in the classification of musical instruments?

Idiophones are defined as instruments that produce sound primarily through the instrument itself vibrating. This means the material of the instrument is the source of the sound, as opposed to needing a separate mechanism, like strings or air, to generate the vibrations. Examples of idiophones include percussion instruments such as xylophones and cymbals, where striking or shaking the instrument causes it to resonate and create sound.

In contrast, the other options describe different categories of instruments. For instance, instruments that require external force to produce sound typically belong to the category of percussion, but this does not capture the essence of idiophones. Bow-stringed instruments fall under the category of chordophones, where vibration comes from strings that are plucked or bowed, and instruments that require air, such as flutes or trumpets, are classified as aerophones. Each of these categories reflects different methods and mechanisms for sound production in musical instruments.