At __________ reflux, theoretical number of plates in a distillation coloumn is calculated using Fenske-Underwood equation.

ANS:B - total

At total reflux, the theoretical number of plates in a distillation column is calculated using the Fenske-Underwood equation. Total reflux refers to the operation of the distillation column where no overhead product is withdrawn, and all of the liquid leaving the condenser is returned as reflux to the top of the column. This condition allows for the most efficient separation of components and serves as a basis for calculating the minimum number of theoretical plates required for a given separation task using the Fenske-Underwood equation.