ANS:D - a counter diffusion at an almost equal molar rate

Binary distillation involves mass transfer by unidirectional diffusion from liquid to gas phase at the gas-liquid interface. During binary distillation, the more volatile component of a liquid mixture preferentially vaporizes and moves from the liquid phase to the vapor phase. This unidirectional diffusion of the more volatile component from the liquid phase to the vapor phase is the primary mechanism by which separation occurs in binary distillation. Therefore, the correct option is unidirectional diffusion from liquid to gas phase.