ANS:D - Fenske-Underwood equation is used for calculating theoretical number of plates in a distillation column at normal operating reflux condition.

The wrong statement is: "Reboiler is not used in open steam distillation." In fact, reboilers are commonly used in open steam distillation processes. In open steam distillation, a mixture of water and the compound to be distilled is heated under reflux, and steam is introduced to carry the volatile components to a condenser for collection. A reboiler is used to provide the necessary heat to vaporize the water and the volatile components. The reboiler ensures continuous vaporization of the liquid mixture, allowing for efficient separation of the volatile components from the non-volatile ones. Therefore, the statement "Reboiler is not used in open steam distillation" is incorrect.