a
Since fluorine is more electronegative than nitrogen, \(H - F\) bonds are much more polar than \(N - H\) bonds. Due to the high partial charges on \(H\) and \(F\) in \(HF\), a Hydrogen bond between \(HF\) molecules is much stronger than that between \(NH_3\) molecules. The high boiling point of \(HF\) is thus due to the strong \(H\)-bond.