Or put another way:
The greater the charge, the higher the lattice energy.
The smaller the size of the ion, the higher the lattice energy.
a) Li+ is smaller in size than Cs+, so lattice energy of LiF is greater than for CsF
b) Br- is smaller than I- so NaBr will have greater LE
c) O2- has a greater charge than Cl-
d) Ca2+ has a greater charge than Na+
e) O2- has a greater charge than F-
f) Li+ is smaller than Na+ and O is smaller than S