Darius strengthened the Persian Empire's economy by introducing a common currency.
Answer: Option D
Step-by-step explanation:
The King of Persia, Darius the Great was a visionary who took certain steps for the strengthening of his Empire's economy. One of these steps was the introduction of a universal currency that could be used to trade across his Empire.
The currency not only gained acceptance in Persia but also transcended the boundaries to reach Eastern Europe. Daric, the universal currency, made the trade very convenient in the region.