Answer
NaF
Work Step by Step
The electrostactic attraction force, by Coulomb's Law is $F=-k\frac{q_1q_2}{d^2}$,
The distance, in this case, is the sum of the ionic radii:
$NaF$: $116\ pm + 119\ pm = 235\ pm $
$NaI$: $116\ pm + 206\ pm = 322\ pm $
Since in both cases one electron is exchanged, in NaF the force of attraction is bigger because the distance between the charges is smaller.
