ANS:B - least at the outlet of the last effect.

In a forward feed multiple effect evaporator, where the vapor from the preceding effect is used as the heating medium for the next effect, the pressure build-up will be highest at the inlet of the last effect. As the vapor moves from one effect to the next, it undergoes pressure drop due to the resistance encountered by the flow path and the condensation process. This results in a gradual decrease in pressure from the inlet of the first effect to the outlet of the last effect. Therefore, the pressure will be highest at the inlet of the last effect. Therefore, the correct option is: highest at the inlet of the last effect.