ANS:A - interface

In absorption and stripping processes, the interface between the liquid and gas phases is where mass transfer occurs, typically involving the transfer of solute molecules from one phase to the other. When the diffusional resistance of the interface is zero, it means that mass transfer across the interface is not limited by diffusion and occurs instantaneously. So, when the diffusional resistance of the interface is zero, the interface of the liquid and gas phases is in equilibrium, ensuring rapid and efficient mass transfer. Therefore, the correct answer is 'a' - interface.