Answer:
Shelly used the harmony principle of design in her screen.
Step-by-step explanation:
Principles of Design are the basic idea that help one to create or produce good visual designs. They are actually fundamental strategies that intend to practice better designs visually.
Harmony is such a principle of design that collects all the parts of visual image together. Each element present in the design complements each other due to harmony. So this can be achieved by repeating and establishing rhythm.
So Shelly should use Harmony in her design.
I hope it will help you!