Assertion (A) $\mathrm{Mg}^{2+}$ and $\mathrm{Al}^{3+}$ are isoelectronic but the magnitude of ionic radius of $\mathrm{Al}^{3+}$ is less than that in $\mathrm{Mg}^{2+}$.

Reason (R) The effective nuclear charge on the outermost electrons in $\mathrm{Al}^{3+}$ is greater than that in $\mathrm{Mg}^{2+}$.The correct option among the following is