Concept:Hydrogen bonds occur when hydrogen is attached to a highly electronegative atom.Explanation:A hydrogen bond is an intermolecular attraction formed when a hydrogen atom is covalently bonded to a strongly electronegative atom like N, O, or F.This electronegative atom pulls the shared electron pair toward itself, leaving the hydrogen atom with a partial positive charge.The partially positive hydrogen can then attract a lone pair of electrons on another electronegative atom in a nearby molecule.Therefore, hydrogen bonding requires a hydrogen atom bonded to a strongly electronegative atom.Answer:A. strongly electronegative atom