ANS:B - decreases

The vapor pressure of liquids generally decreases with an increase in molecular weight. This is because higher molecular weight substances typically have stronger intermolecular forces (like van der Waals forces) which hold the molecules together more tightly. As a result, fewer molecules are able to escape from the liquid phase to form vapor, leading to lower vapor pressure compared to substances with lower molecular weights. Therefore, the correct answer is: (b) decreases