Which of the hydrogen compounds has the highest percentage of ionic character?

Explanation

The ionic character of a bond is largely determined by the difference in electronegativity between the two bonded atoms. Fluorine has the highest electronegativity of all elements, resulting in the largest electronegativity difference between hydrogen and fluorine in HF. This large difference gives HF the highest percentage of ionic character among the given hydrogen compounds. In contrast, HCl, HBr, and HI have progressively lower electronegativity differences and hence lower ionic character, with HI having the lowest due to iodine's relatively low electronegativity.