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