Iodine is a molecular solid composed of I2 molecules. The intermolecular forces between I2 molecules are weak van der Waals forces. The boiling point of a substance is the temperature at which its vapor pressure equals the pressure of the surrounding atmosphere. The strength of the intermolecular forces determines the boiling point of a substance. The weaker the intermolecular forces, the lower the boiling point. Therefore, iodine has the lowest boiling point among the given molecular solids.