Which acid is used to acidify the solution in titrations with $\mathrm{KMnO}{}_4$?

$\mathrm{HCl}$ is oxidised by permanganate (to $\mathrm{Cl}{}_2$) and $\mathrm{HNO}{}_3$ is itself an oxidant, so both interfere. Acetic acid is too weak to supply enough $\mathrm{H}{}^{+}$. Dilute $\mathrm{H}{}_2\mathrm{SO}{}_4$ is neither oxidised nor oxidising and is used.