Answer
$RbI\lt LiI\lt LiF\lt CaO$
Work Step by Step
$RbI\lt LiI\lt LiF\lt CaO$
Lattice energy is the energy released by combining ions in a lattice, therefore it increases with smaller ions because they are closer, and with greater charges, because the attraction is greater.
Rubidium is in period 5, so it has the largest cation radius and therefore the lowest lattice energy.
Fluorine has a smaller atomic radius than iodine.
Calcium and oxygen have positive and negative charges respectively of magnitude 2.
