The solubility of which of the following sparingly soluble salts is increased most by lowering the pH of the solution?

Carbonate is the conjugate base of a weak acid: $\mathrm{CO}{}_3{}^{2-}+\mathrm{H}{}^{+}\to \mathrm{HCO}{}_3{}^{-}$ (and further to $\mathrm{H}{}_2\mathrm{CO}{}_3$), so added acid removes the anion and shifts $\mathrm{CaCO}{}_3\rightleftharpoons \mathrm{Ca}{}^{2+}+\mathrm{CO}{}_3{}^{2-}$ to the right. The anions $\mathrm{Cl}{}^{-}$ and $\mathrm{I}{}^{-}$ are conjugate bases of strong acids, and $\mathrm{SO}{}_4{}^{2-}$ is a very weak base, so the others change little.