When ${\mathrm{KMnO}}_4$ solution is added to hot oxalic acid solution, the decolorization:, is slow in the beginning but becomes instantaneous after some time. This is because :

In this case, ${\mathrm{KMnO}}_4$ is a reactant in the process that produces ${\mathrm{Mn}}^{2+}$, which works as an auto-catalyst. The reaction is sluggish at first because ${\mathrm{KMnO}}_4$ is being converted to ${\mathrm{Mn}}^{2+}$, but when ${\mathrm{Mn}}^{2+}$ forms, the process speeds up owing to catalytic activity.

Hence, option A is correct.