The first ionisation energy of magnesium is smaller as compared to that of elements $X$ and $Y$, but higher than that of $Z$. The elements $X, Y$ and $\mathrm{Z}$, respectively are:

Due to an increase in nuclear charge, the order of ionisation energy increases over time.

$3^{rd}$ period $\to \mathrm{Na}<\mathrm{Al}<\mathrm{Mg}<\mathrm{Si}<\mathrm{S}<\mathrm{P}<\mathrm{Cl}<\mathrm{Ar}$

As a result, magnesium will have lower initial ionisation energy than chlorine and argon.

Lithium and sodium will have lower initial ionisation energies than magnesium because they will have a stable electronic structure after losing an electron.

Hence, the required option is C) argon, chlorine and sodium.