Final answer:
The harp is not a percussion instrument; it is a chordophone, which is a class of stringed instruments. Percussion instruments include the timpani, xylophone, and snare drum.
Step-by-step explanation:
The instrument that is NOT a percussion instrument among the options provided is the harp. The timpani, xylophone, and snare drum all fall under the category of percussion instruments. The timpani are a type of drum, specifically a kettle drum with a pitch that can be altered. The xylophone is an idiophone, as it produces sound from the material itself without needing strings or a membrane. The snare drum is a membranophone, which has a drumhead that produces sound when struck.
Meanwhile, the harp is a chordophone because it produces sound from the vibration of its strings when they are plucked. In the common classification system used in ethnomusicology, chordophones are identified as stringed instruments distinct from the percussion family.