Greek city-states had supported rebellions in the Persian Empire. This caused Darius to decide to attack Greece following which, the Persian wars started.
Answer: Option C
Explanation:
The fourth king of the Achaemenid Empire, Darius I, was angrier by the support that was offered by the Greek city-states of Eretria and Athens to the Persian cities of Ionia when they had rebelled against the throne of Darius I.
Following the rebellion, Darius I decided to invade Greece. That resulted in the series of war between Persia and the Greece which are known as Persian wars.