Final answer:
The strength of dispersion forces depends on the molecular weight and surface area, as larger molecules with more electrons exhibit stronger dispersion forces, which can affect the physical state of a substance at room temperature.
Step-by-step explanation:
The strength of dispersion forces in molecules is determined by various factors, among which molecular weight and surface area are key. Dispersion forces are a type of intermolecular force that occur due to temporary dipoles in molecules, created when electrons are distributed asymmetrically. These forces are weak and only become significant when molecules are very close to each other. Larger and heavier atoms and molecules have more electrons, thus displaying stronger dispersion forces than their smaller and lighter counterparts. This explains why some substances are gases at room temperature, while others with stronger dispersion forces are liquids or solids.
The correct answer to the student's question, "The strength of these forces depended on two factors", is (c) Molecular weight and surface area, as these two factors significantly influence the strength of dispersion forces between molecules.