The answer to this question would be: lif
percent ionic character is used to determine if a bond is more ionic or covalent. The more percent ionic character, the more ionic the molecule is. If the electronegativity difference(delta E.N.) between two elements is higher than 1.7 it will be considered more ionic than covalent.
If you count the delta E.N. of those molecules the number should be 1.9 for HF, 0.9 for HCl, 0.7 for HBr, and 3.0 for LiF.
In this case, lif would be the one with highest percent ionic character.
