Final answer:
Diagonals are the design element that represents the directional wave patterns or illusion of motion in the hair.
Step-by-step explanation:
The design element that represents directional wave patterns or the illusion of motion in the hair is Diagonals. Diagonals create a strong zigzag pattern that conveys movement. In art and design, diagonal lines are often used to create a sense of energy, dynamism, and direction. They can be used to guide the viewer's eye and create a sense of movement.