The correct answer is:
d. color
Explanation:
1. Proportion, emphasis, and rhythm are all principles of design in art.
2. Color, on the other hand, is not typically considered a principle of design; instead, it is often categorized as an element of design.
3. Color is a fundamental element used in art to evoke emotions, create visual interest, and convey meaning, but it is not classified as a principle of design.