Final answer:
Intermolecular forces directly affect a substance's boiling point and melting point because they determine how much energy is needed for the substance to change state.
Step-by-step explanation:
The type and magnitude of intermolecular forces present in a substance significantly influence several of its physical properties, including boiling point, melting point, and density. However, they do not determine the substance's molar mass.
When considering the options provided, both boiling point and melting point are most directly determined by intermolecular forces. This is because a substance transitions from one state to another (solid to liquid or liquid to gas) when its molecules acquire enough thermal energy to overcome the intermolecular forces holding them in the respective state.
For instance, a substance with relatively strong intermolecular forces will generally have a higher melting point and boiling point because more energy is required to break the forces holding the molecules together.