Answer:
d. civil war
Step-by-step explanation:
The Persian Empire spanned through several countries in and around the modern Iran. It was a global hub of culture, religion, science, art and technology before it was conquered by Alexander the Great. After a failed invasion of Greece by Xerxes I, Persia started to decline. In order to strive it had to raise taxes which then caused rebellion among people, against its rulers. Even though there was a threat of invading armies of Alexander the Great, there were constant plots to take over the throne which resulted in the rising weakness of the rulers which made it easy for Alexander the Great to regain Greek territories and enlarge his empire all the way to India.