Which quality is most lost if a glass vessel is outlined only with a continuous black line?

Correct answer: Transparency and variation in light-based edges

The governing concept is that transparent glass is communicated through changing edges, reflections, refractions, and visible overlaps rather than a uniform outline. A continuous heavy black contour makes every boundary equally strong and suggests an opaque, cut-out object. Option D is correct because it removes the delicate transparency and variation caused by light and background. Symmetry and height can still be drawn, and a cast shadow is a separate tonal feature that need not disappear.