Which of the following is the formula of magnesium salt of ‘$X$’ if the aluminium salt of anion ‘$X$’ is ${\mathit{Al}}_2{X_3}_{ }$?

$\mathit{MgX}$ is the formula of magnesium salt of ‘$X$’ if the aluminium salt of anion ‘$X$’ is ${\mathit{Al}}_2{X_3}_{ }$ .
Valency is defined as the number of electrons present in the atom's valence shell. The valency of metal in the formula of ionic salts is written at the subscript of non-metal and vice-versa. So the valency of $X$ in ${\mathit{Al}}_2{X_3}_{ }$ is $2$. 

The valency of Magnesium is $2$.

So, the formula of magnesium salt will be $\mathit{MgX}$.