Solution

Magnesium is an alkaline earth metal and forms $$\displaystyle Mg^{2+}$$ ion.

The element $$X$$ with ground state electronic configuration of $$1s^{2} 2s^{2} 2p^{3}$$ will form $$\displaystyle X^{-3}$$ ion.

The charges on cation and anion will be balanced when 3 $$\displaystyle Mg^{2+}$$ ions will combine with 2 $$\displaystyle X^{-3}$$ ions to form $$\displaystyle Mg_3X_2$$

Hence, the correct option is C.