Answer: MgO
Explanation: In both CaO and MgO, the charges are +2 and -2, so the choice is between these two oxides.
The smallest ions will be able to get closest to each other.
They will have the smallest distance between centres and will have the largest lattice energies.
The smallest ions are at the top of the Periodic Table.
Mg
2
+
is smaller than
Ca
2
+
, so
MgO
has the largest lattice energy.