All the alkali metals when heated with oxygen form different types of oxides for example lithium forms lithium oxide (Li₂O), sodium forms sodium peroxide(Na₂O₂), while K, Rb and Cs form their respective superoxides.

Lattice energy decreases more rapidly than hydration energy for alkaline earth metal hydroxides.On moving down a group.solubility of their hydroxides increases.

Smaller the ion more is its ionic mobility in aqueous solution. Ionic radii of the given alkali metals is in the order Na⁺ < K⁺ < Rb⁺ < Cs⁺ and thus expected ionic mobility will be in the order Cs⁺ < Rb⁺ < K⁺ < Na⁺.

However due to high degree of solvation (or hydration)because of lower size or high charge density, the hydrated ion size follows the order Cs⁺ < Rb⁺ < K⁺ < Na⁺ and thus conductivity order is Cs⁺ > Rb⁺ > K⁺ > Na⁺ i.e. option (b) is correct answer.

As the basicity of metal hydroxides increases down the group from Be to Ba, the thermal stability of their carbonates also increases in the same order. Further group 1 compounds are more thermally stable than group 2 because their hydroxide are much basic than group 2 hydroxides therefore, the order of thermal stability is BeCO₃ < MgCO₃ < CaCO₃ < K₂CO₃.