The correct answer is: "the Mediterranean Sea".
The capital of the Empire, Rome, was initially located on a very strategical point, in the middle of the Northern shore of the Mediterranean sea. Moreover, the Roman conquerors expanded the territories of the Empire around the Mediterranean coasts. It was easy to conduct internal ship trade among Roman provinces, and also to undertake commercial activities with other Mediterranean civilizations such Phoenicians, in the North African coasts, or the Carthaginians. These rich trade activities, together with the territorial expansion, gave economic glory to the Roman Empire, and in turn, power.