Among F,O and N,F is most electronegative so F pulls bond pair of electron in F - H towards itself and develops highly positive charge on H atom. This highly positive charged H atom creates strongest hydrogen bonding by taking lone pair of electron form electronegative atom F/N/O. Hence among the given options F−H......F is the strongest bond. Note : F−H......N has strongest hydrogen bonding among F−H......F,F−H......O and F−H...... N because N is least electronegative among F,O and N and can easily donate lone pair of electron.