JEE Chemistry Practice Question

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

1. A.3
2. B.4
3. C.5
4. D.2