(a) Since the halides are a polar molecule (due to high electronegativity), due to the presence of permanent dipoles, the dipole-dipole interactions along with the London forces are found in HF, HCl and HBr.
(b) The boiling point increases from HI to HCl (as I– (iodide ion) is a very large ion and is bonded by Van der Waal’s forces, so more energy is required to break the bonds. As the size of the ions decreases, lesser energy is required). From this trend, we can conclude that London’s forces are predominant because these interactions also increase from HF to HI.
(c) In HCl, size of Cl– ion is very small, due to which the bonded Van der Waal’s forces are very weak, so less energy is required to break the bonds. Thus the boiling point is the least.