ANS:B - evaporation

'Duhring's plot' is of use in extractive distillation. Duhring's plot is a graphical method used in extractive distillation to determine the composition of the overhead vapor and the liquid residue in the distillation column. It helps in optimizing the design and operation of extractive distillation processes by providing insights into the separation efficiency and the concentration profiles of the components in the column. Therefore, the correct option is: extractive distillation.