Identify the elements $X$ and $Y$ using the ionisation energy values given below:

Ionisation energy $(\mathrm{kJ}/\mathrm{mol})$

	1st	2nd
X	495	4563
Y	731	145

Due to $2p^6$, noble gas electronic configuration, the second ionisation enthalpy of $\mathrm{Na}$ is very high. That's why has large difference between ${\mathrm{IE}}_1$ and ${\mathrm{IE}}_2$ of ${\mathrm{Mg}}^{+}$ is $2{\mathrm{p}}^6, 3{ \mathrm{s}}^1$.

After the loss of one electron, ${\mathrm{Mg}}^{2+}$ will be formed with a noble gas electronic configuration. That's why there is less difference between ${\mathrm{IE}}_1$and ${\mathrm{IE}}_2$.

Hence, the required option is A.