ANS:C - binary phase distillation

Steady-state equimolal counter-diffusion occurs in binary phase distillation. In binary phase distillation, two components with similar volatility are being separated. As they travel up the distillation column, there's a continuous exchange of molecules between the liquid and vapor phases. Equimolal counter-diffusion means that the rate of transfer of each component from the liquid phase to the vapor phase and vice versa is equal. This results in no net change in the composition of the liquid or vapor phase, which is characteristic of equimolal counter-diffusion. The other processes mentioned—leaching, absorption, and liquid-liquid extraction—involve different mechanisms and are not typically associated with equimolal counter-diffusion.