For which pH change is there the maximum increase in acidity?

4.

6

2

Reason — According to the pH scale, lesser the pH more is the acidity. When pH decreases from 6 to 2, the amount of H⁺ increases 10,000 times.

pH= −log₁₀ [H⁺ ] ⟶ [H⁺ ] = 10^-pH ​

Initial [H⁺] = 10^-6 = 0.000001 mol/L
Final [H⁺] = 10^-2 = 0.01 mol/L

Change: $\frac{0.01}{0.000001}$ = 10,000 times more acidic