Which compound will have the larger magnitude enthalpy of hydration?

a.
The answer: NaCl

In aqueous solutions, NaCl dissociates to give Na⁺ and Cl⁻ ions, while CsCl dissociates to give Cs⁺ and Cl⁻ ions.
Na⁺ has a smaller cationic size than Cs⁺. Hence, the distance between Na⁺ ions and water molecules is less than that between Cs⁺ and water molecules. This leads to a result that the attractive forces between Na⁺ ions and water molecules are stronger than those between Cs⁺ ions and water molecules. Hence, the hydration of Na⁺ ions is more exothermic than that of Cs⁺ ions.

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b.
The answer: Mg(NO₃)₂

In aqueous solutions, NaNO₃ dissociates to give Na⁺ and NO₃⁻ ions, while Mg(NO₃)₂ dissociates to give Mg²⁺ and NO₃⁻ ions.
Since Mg²⁺ ion carries a higher positive charge and has a smaller size than Na⁺ ion, the attractive forces between Mg²⁺ ions and water molecules are stronger. Hence, the hydration of Mg²⁺ ion is more exothermic than that of Na⁺ ion.