ANS:B - backward

For the evaporation of a viscous solution in a multiple effect evaporator, the preferred feeding scheme is typically backward or counter-current feeding. In backward or counter-current feeding, the feed is introduced into the last effect of the evaporator system, and the concentrated solution flows in the opposite direction to the heating medium. This arrangement provides several advantages, including maximizing the temperature gradient across each effect, minimizing the temperature difference between the feed and the heating medium, and enhancing heat transfer efficiency. So, the correct answer is: "backward"