The correct option is "A dominating nation with extensive territories and a powerful ruler such as Rome's is known as an empire."
The Roman Empire was the third period of Roman civilization in classical antiquity, after the Roman Republic and characterized by an autocratic form of government. The birth of the Empire is preceded by the expansion of its capital, Rome, which extended its control around the Mediterranean Sea. Under the imperial stage the domains of Rome continued to increase until they reached their maximum extent during the reign of Trajan, at which time it extended from the Atlantic Ocean to the west to the banks of the Caspian Sea, the Red Sea and the Persian Gulf to the east, and from the Sahara desert to the south to the wooded lands on the banks of the Rhine and Danube rivers and the Caledonian border to the north. Its estimated maximum area would be about 6.5 million km².