The alkaline earth metal nitrate that does not crystallise with water molecules, is :

$\mathrm{Sr}\left(\mathrm{NO}_{3}\right)_{2}$

$\mathrm{Mg}\left(\mathrm{NO}_{3}\right)_{2}$

$\mathrm{Ca}\left(\mathrm{NO}_{3}\right)_{2}$

$\mathrm{Ba}\left(\mathrm{NO}_{3}\right)_{2}$