JEE Advanced 2023 Chemistry question on Equilibrium

On decreasing the $p$H from 7 to 2, the solubility of a sparingly soluble salt (MX) of a weak acid (HX) increased from $10^{-4}$ mol L$^{-1}$ to $10^{-3}$ mol L$^{-1}$. The $p$K$_a$ of HX is