ANS:C - sum of (a) and (b).

The heat of adsorption of a gas caused by van der Waals forces of attraction and capillarity is akin to the process of physisorption, where gas molecules adhere to the surface of a solid due to weak van der Waals forces.

1. Normal condensation: This refers to the process of a gas transforming into a liquid under normal conditions. It's typically associated with stronger intermolecular forces compared to physisorption.
2. Wetting: Wetting involves the spreading of a liquid over a surface, driven by intermolecular interactions between the liquid and the surface. This is a phenomenon distinct from gas adsorption.

Since the heat of adsorption caused by van der Waals forces and capillarity is associated with gas adsorption, and it's a weaker interaction compared to normal condensation, it's reasonable to say that the heat of adsorption is less than the heat of normal condensation. Therefore, the correct option is difference of (a) and (b).