The right answer is "the institution of a single language for government ".
Darius organized the empire, dividing it into provinces (satrapias), and putting satraps to govern them. He organized a new unified monetary system, and made Aramaic the official language of the empire. He also instituted construction projects throughout the empire, especially in Susa, Pasargada, Persepolis, Babylon and Egypt - where he was also responsible for codifying the laws.