Lattice energy is essentially ionic bond strength. Some Text books define it as the energy required to break an ionic bond which is a positive value while others describe it as the energy released when ionic bond forms from gaseous ions which is a negative value. The question wants us to put them in order from strongest to weakest (decreasing) bond because the word magnitude is used.
To completely understand how to answer this question requires us to apply coulombs law. This law deals with the strength of an attraction between charged particles, at least particles of opposite charge. The strength of an ionic bond is directly proportional to the product of the charges of the ions and inversely proportional to the distance between them. So the strongest ionic bonds form between ions with large charges on small ions.
KF, +1 and -1
RbI, same, and these ions are larger than K+ and F- since Rob and I are lower on the periodic table.
BaO, +2 and -2 !!!!!
BaO, barium oxide, contains more highly charged ions, the product of +2 and -2 is larger than +1 and -1. The size difference is also small between K and F, the next smallest ions.
So BaO followed by KF followed by RbI, choice E.
