Which lighting technique highlights the three-dimensional form of a subject?

The key light is fundamental in photography as it is the primary source of light used to illuminate a subject. It serves to create highlights and shadows, which are essential for defining the three-dimensional form of the subject. When strategically positioned, the key light accentuates contours and textures, providing depth and dimensionality to the image. By controlling the intensity and angle of the key light, photographers can manipulate how light falls on the subject, thereby enhancing its volume and shape.

In contrast, soft lighting diffuses light to reduce shadows, which may flatten features and is less effective in creating a strong three-dimensional effect. Back lighting illuminates a subject from behind, often creating silhouettes and highlights around the edges but can obscure details in the front. Panning lighting refers to a motion technique rather than a specific lighting setup, often used in sports photography to capture movement. Thus, the key light is the most effective option for highlighting a subject's three-dimensional form.