Bharat Muni, in his Natya Shastra, categorises musical instruments into four different parts. What is the categorisation of musical instruments by Bharat Muni based on?

The correct answer is Different sounds of the musical instruments.

In ancient Indian musicology, Bharat Muni, in his seminal treatise the Natya Shastra (Chapter 28), introduced a comprehensive four-fold classification system for musical instruments (collectively referred to as Atodya or Vadya). This system categorizes instruments fundamentally based on their acoustic principles and the different sounds they produce:

1. Tata Vadya (Chordophones): Stringed instruments where sound is produced by the vibration of stretched strings, played either by plucking, striking, or bowing (e.g., Veena, Ektara, Sitar).

2. Avanaddha Vadya (Membranophones): Percussion instruments where sound is generated by striking a stretched animal hide or membrane over a hollow body (e.g., Mridangam, Tabla, Dholak).

3. Ghana Vadya (Idiophones): Solid percussion instruments that generate sound through the vibration of the solid material itself when struck or clashed together, without requiring stretched strings or membranes (e.g., Manjira, Jalra, Bells).

4. Sushira Vadya (Aerophones): Wind instruments where sound is created by blowing air into a hollow column or pipe to create acoustic resonance (e.g., Bansuri/Flute, Shehnai).

Thus, the 4-fold categorization established by Bharat Muni is based on the different sounds and acoustic methods of sound production of the musical instruments.