ANS:C - ∞

The number of theoretical plates in a distillation column is minimized at a reflux ratio of ∞. At a reflux ratio of ∞, all the vapor leaving the top of the column is condensed and returned to the column as reflux. This essentially means that no product is withdrawn from the top of the column, and all the vapor is condensed and returned to the column. In this scenario, the equilibrium between the liquid and vapor phases is constantly maintained throughout the column, resulting in the minimum number of theoretical plates required for separation. Therefore, the correct answer is ∞.