Question

What is the vibrating part of Harp Musical instrument

Answer 1

https://en.m.wikipedia.org/wiki/String_instrument
String instrument - Wikipedia

Answer 2

Explanation:

You have plucked a string, it has driven energy into the soundboard, the soundboard is vibrating and the back of the harp is reinforcing that vibration. This means that the front surface of the soundboard is vibrating nicely. As it does this, just as with the inside, the soundboard is pushing and pulling air.