ANS:D - both (b) and (c)

The heat of vaporization generally decreases with an increase in pressure. This is because at higher pressures, the molecules in the liquid phase are more tightly packed, and more energy is required to overcome the increased intermolecular forces to transition from the liquid phase to the vapor phase. However, at very high pressures approaching the critical point, the distinction between the liquid and vapor phases becomes less distinct, and the heat of vaporization approaches zero. This occurs at the critical pressure and temperature, where the distinction between the liquid and vapor phases disappears, and the substance exhibits properties of both phases simultaneously. So, the correct completion of the statement is: both (b) and (c)