Answer:
The correct answer is the option A: gradation.
Explanation:
On one hand, the<em> design elements</em> are the fundamental ideas about the practise of the visual design, which includes the principles of gradation, contrast, alignment, balance and more. <em>These elements help the person's work to be more pleasant to the eye and better</em>.
On the other hand, <em>gradation</em> is the name given to a principle of design whose main characteristic is in the <em>gradual changes</em> done in the art elements with the purpose of adding more value to the work. Moreover, it basicaly refers to <em>step-by-step changes</em>, that could be for example from dark to light or from large to small shapes.