Which effect decreases most if all values become middle grey after blending?

Correct answer: Value contrast and volume

Form is communicated through a range of values: highlights, middle tones, and darker areas. If blending reduces every area to middle grey, the difference between light and dark disappears. Without that value contrast, the viewer receives much less information about planes, curvature, and volume. The contour, proportion, and paper texture may still exist, but they are not the main effect lost. Hence, option D is correct.