The correct order of thermal stability of hydroxides is

$Mg{\left(OH\right)}_{2} < Ca{\left(OH\right)}_{2} < Sr{\left(OH\right)}_{2} < Ba{\left(OH\right)}_{2}$

Thermal stability increases down the group due to the decrease in the charge density of the metal cation. All group 2 metal ions have the same charge (2+) but the smaller ones at the top of the group have it all packed into a smaller space, so they polarise the O-H bond more (pull electrons towards the oxygen). This means the O-H bond is already partly broken, so it requires less heat to break fully. As you go down the group, the O-H is less polarised by the larger group 2 cations with a lower charge density, so the thermal stability increases.