Constantine moved the Roman capital from Rome to this city in the eastern part of the empire, which he renamed Constantinople.

The correct answer is a. Byzantium.

Constantine moved the Roman capital to Byzantium and renamed it Constantinople in AD 330. He chose this location due to its strategic position for trade and defense, as it sat at the crossroads of Europe and Asia. This city later became a central hub of culture and power in the Eastern Roman Empire, also known as the Byzantine Empire.