Among the following slowest reaction under identical conditions is

In reaction (A), ${\mathrm{H}}^{+}$ and ${\mathrm{OH}}^{-}$ are combining to ${\mathrm{H}}_2\mathrm{O}$. It is a very fast reaction.

In reaction (B), the ${\mathrm{KMnO}}_4$ reacts as oxidizing agent while the $\mathrm{C}-\mathrm{C}$ bond of the oxalic acid breaks. But the $\mathrm{C}-\mathrm{C}$ bond in oxalic acid is a very strong bond due to which the reaction occur at a slow rate.

In reaction (C), the ${\mathrm{KMnO}}_4$ changes ${\mathrm{Fe}}^{2+}$ to ${\mathrm{Fe}}^{3+}$. The change of ${\mathrm{Fe}}^{2+}$ to ${\mathrm{Fe}}^{3+}$ is a very fast reaction.

Reaction (D) is a precipitation reaction because $\mathrm{AgCl}$ is precipitated and it is a very fast reaction.