The pair of ions for which the energy that is required to separate the ions largest is Mg²⁺ and O²⁻.
Lattice energy is the energy released when ions combine to form the lattice structure of an ionic compound.
Lattice energy depends on the size of the both ions. Smaller ions have a larger lattice energy and a largest energy required to separate the ions.
Looking at the options, the both smallest pair of ions is Mg²⁺ and O²⁻ hence they have the highest lattice energy and largest energy to separate the ions.
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