To determine the thermal stability order of $H_2E$ (where $E = O, S, Se, Te,$ and $Po), \\$we need to consider the bond strength between hydrogen and the chalcogen (E).$\\$The thermal stability of these hydrides is primarily influenced by the bond dissociation energy.$\\$As we move down the group in the periodic table, the size of the chalcogen atom increases.$\\$This leads to a decrease in bond strength due to poorer overlap between the hydrogen 1s orbital$\\$and the larger, more diffuse orbitals of the heavier chalcogens.$\\$Therefore, the bond strength decreases as we move from $H_2O$ to $H_2Po. \\$This results in a decrease in thermal stability in the same order.$\\$Thus, the correct thermal stability order is:$\\ H_2Po < H_2Te < H_2Se < H_2S < H_2O \\$This corresponds to Option 3.