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

The successive ionisation energy values for an element ' $X$ ' are given below :

(i) 1st ionisation energy $=410 \mathrm{~kJ} \mathrm{~mol}^{-1}$

(ii) 2nd ionisation energy $=820 \mathrm{~kJ} \mathrm{~mol}^{-1}$

(iii) 3rd ionisation energy $=1100 \mathrm{~kJ} \mathrm{~mol}^{-1}$

(iv) 4th ionisation energy $=1500 \mathrm{~kJ} \mathrm{~mol}^{-1}$

(v) 5th ionisation energy $=3200 \mathrm{~kJ} \mathrm{~mol}^{-1}$