The statement that best describes the beliefs of Zoroastrianism is: "Good and evil constantly struggle for control of the universe."
Additionally, Zoroastrianism also teaches that there is one supreme god, Ahura Mazda, who is opposed by an evil spirit, Angra Mainyu (Ahriman). Therefore, the statement "There is just one supreme god, but he has an evil opposite" is also accurate in the context of Zoroastrianism.
Overall, Zoroastrianism emphasizes the duality of good and evil and the role of human agency in choosing between them.