The correct answer is D. All of the following were contributing factors to the fall of the Persian Empire except civil war.
Options A, B and C are correct. Persian throne was a source of constant conspiracies to access to it, which caused bloody internal clashes. For example, the successor of Artaxerxes II, his son Artaxerxes III, came to the throne by bloody ways, securing his position by killing eight of his half brothers. In 343 a. C., Artajerjes III defeated to Nectanebo II, expelling it of Egypt and made of Egypt again a Persian satrapy. When the economic decline became evident, the Persian Empire raised taxes, which further convulsed society.
And of course, the final blow to the Persian Empire was the Greek invasion, carried out by Alexander the Great. In the year 331 a. C., the Macedonian army invaded Persia easily entering Susa, the old capital of Darío I, whereas the defeated Darío III fled towards the interior of the Persian territory in search of loyal forces to face again to Alexander.
Alexander proceeded carefully to occupy the cities, seizing the Persian flows and securing the supply lines. From Susa he went to Persepolis, the ceremonial capital of the Achaemenid Empire, where he burned the city palace during a party. Then they headed towards Ecbatana to pursue Darío. He was found murdered by his nobles, who now obeyed Bessos. Later, Alexander killed Bessos and was crowned as King of Persia.
