Step 1: Understanding the Act.
The India International Arbitration Centre Act, 2019 was enacted to establish an independent and autonomous arbitration institution in India.
Step 2: Statutory description.
The Act expressly declares the India International Arbitration Centre to be an institution of national importance, highlighting its significance in strengthening arbitration in India.
Step 3: Elimination of incorrect options.
The statute does not describe the Centre as an institution of public, commercial, or economic importance in explicit terms.
Step 4: Conclusion.
Hence, the correct answer is (C) national.