strength of ionic bond is proportional to the product of charges of the cation and anion, and inversely proportional to the sum of radius of the cation and anion.
(q+ × q-) / (r+ + r-)
by comparing the radius of the ions of similar charges, strength of ionic bond: LiF > NaBr > KI
next, comparing BaCl2 and LiF,
the charge of Ba2+ > Li+, however the radius of Ba2+ and Cl- >> Li+ and F-
hence LiF has the strongest ionic bond