What visual cue gives away the curves of someone wearing a glossy black jacket?

Answer: Specular highlights from the glossy surface trace the curves with bright streaks.

Specular highlights from the glossy surface trace the curves with bright streaks. ✓ — Right. Specular highlights are bright reflections on glossy surfaces, and vision research shows they can provide information about 3D shape. A black garment can still reveal curvature if its surface creates strong highlights. The key distinction is color versus surface reflection: dark color reduces overall returned light, but glossy reflection can still draw shape-revealing streaks.

Darker pigment absorption creates shadow outlines that reveal the curves. — Not quite. Darker pigment absorption can deepen shadows, but it does not explain a bright streak that follows surface curvature. The shape cue here is reflected highlight structure, not pigment darkness. A surface can be very dark in color yet still produce a highlight if it is glossy.

Matte surface scatter softens light into shading that reveals the curves. — Not quite. Matte surface scatter is the opposite family of cue from the glossy highlight that gives the curves away. Matte scatter can give some soft shading, but the shape information here comes from specular highlights. The important lesson is that blackness and gloss are different visual properties: a black surface can hide or reveal depending on reflection structure.