The s-Block Elements

D.

LiH > NaH > KH >RbH >CsH

Small anion forms stable compounds with small cation.
The thermal stability of alkali metal hydrides decreases as:
LiH> NaH > KH > RbH  >CsH
because the size of cation increases as;
Li⁺ < Na⁺ < K⁺ < Rb⁺ <Cs⁺

B.

BeCO₃ < MgCO₃ < CaCO₃ <K₂CO₃

Thermal stability of carbonates increases in a group as we move from top to bottom and decreases in a period as we move from left to right. so, the correct order of thermal stability of given carbonates is:
BeCO₃ < MgCO₃ < CaCO₃ < K₂CO₃
Be, Mg and Ca present in second group and K present in the first group.

B.

MgSO₄

Hydration energy of sulphate decreases from top to bottom in an IInd group. Mg²⁺ is smaller than other ions of that group so Mg²⁺ is readily hydrated. MgSO₄ has higher hydration energy than lattice energy.

D.

Rb⁺ > K⁺ > Na⁺ > Li⁺

The correct order of the mobility of the alkali metal ions in aqueous solutions 
Rb⁺ > K⁺ > Na⁺ > Li⁺ due to following order of hydration energy of these ions Li⁺ > Na⁺ > K⁺ > Rb⁺ and due to hydration of ion, mobility decreases.