Final answer:
Hydrogen fluoride (HF) should have the highest boiling point because it has the strongest intermolecular forces in the form of hydrogen bonds, which are stronger due to the higher electronegativity of fluorine compared to oxygen in ethanol.
Step-by-step explanation:
To determine which of the listed substances - hydrogen fluoride (HF), methane (CH₄), ethanol (C₂H₅OH), carbon dioxide (CO₂) - should have the highest boiling point, one must consider the type and strength of intermolecular forces present in each substance. The strongest intermolecular forces result in higher boiling points. Hydrogen fluoride (HF) has hydrogen bonding, which is significantly stronger than the London dispersion forces present in methane (CH₄) and carbon dioxide (CO₂). Moreover, although ethanol (C₂H₅OH) also has hydrogen bonding, HF's hydrogen bonds are stronger due to the higher electronegativity of fluorine compared to that of oxygen. Therefore, HF would have the highest boiling point among these substances.