This answer is a very good answer although it is not correct. You are right in thinking that HF can form the strongest hydrogen bonds because the hydrogen atom in HF has more positive character than the hydrogen atoms in the other choices. The other factor that must be considered is the network formed by the hydrogen bonds. This molecule has 3 pairs of non-bonding electrons and only 1 electropositive hydrogen atom. It cannot form a continuous network of hydrogen bonds and thus has a lower boiling point than one of the other choices.