ANS:A - area between operating line and 45° diagonal x-y diagram.

Increasing the reflux ratio above the optimum in binary distillation does not result in an increase in the area between the operating line and the 45° diagonal on the x-y diagram. This is because the optimal reflux ratio is the point at which the area between these lines is maximized, indicating the maximum theoretical separation efficiency. Beyond this point, further increases in reflux ratio do not contribute to additional separation and therefore do not increase this area.