Final answer:
Unity and variety is the design principle that refers to units that are opposite, creates variety, and stimulates interest.
Step-by-step explanation:
The design principle that refers to units that are opposite, creates variety, and stimulates interest is unity and variety. In art, unity creates a sense of completeness and cohesiveness, while variety adds changes and awareness to the art piece. Colors can provide unity when they are in the same color groups, and a splash of a different color can provide variety.