Which of the following is thermally more stable

Thermal stability order can be given as,

BeO₂ < MgO₂ < CaO₂ < SrO₂ < BaO₂
The order of peroxides of the group number 2  in parodic table increases down the group because the radius of cation increases.

Thus, compounds which are thermally more stable than CaO₂ are BaO₂ and SrO_2.