ANS:C - number of plates required will be maximum.

For a distillation column operating at minimum reflux: The concentration of liquid and vapor leaving a plate will be the same. At minimum reflux conditions, the reflux ratio is minimized, and the column operates with the least amount of reflux possible while still maintaining separation efficiency. This results in a situation where the concentrations of liquid and vapor leaving each plate in the column are equal. This is because minimum reflux implies that the amount of liquid returning to the column from the condenser is minimized, leading to similar compositions for the liquid and vapor phases leaving each stage. Therefore, the correct statement is that the concentration of liquid and vapor leaving a plate will be the same.