Explanation:

- The Union Budget of 2021-22 announced several health-related infrastructure developments.

- Among these, it was specified that several new regional National Institutes of Virology (NIV) would be established to strengthen virology research and preparedness for future pandemics.

- Option 1: Four

- The correct answer is Four.

- Four new regional National Institutes of Virology will be set up as part of these initiatives.

- Option 2: Two

- Two new institutes are not sufficient for the budget’s highlighted plans.

- Option 3: Three

- Three institutes fall short of the announced figure.

- Option 4: Six

- Six institutes exceed the planned number.