Decreasing order of the boiling points of compounds will be, H2O>HF>NH3; due to the strength of hydrogen bonding present in given compound. So, more the hydrogen bonding, higher will be boiling point. Each water molecule can potentially form four hydrogen bonds with surrounding water molecules. Due to higher partial charge on H and F in HF, a hydrogen bond between HF molecules is stronger than that of NH3 molecules. Hence, the option (b) H2O>HF>NH3 is correct.