Option A is the right answer that the Black Sea bordered the Eastern Roman Empire.
The Black Sea is an inland sea between Anatolia and Southeastern Europe, which shares its border with several countries including Romania, Turkey, Bulgaria, Russia and Georgia, Ukraine.
This Sea gain more importance after the foundation of the City of Constantinople, which became the first capital of the Eastern Roman Empire and later the Byzantine Empire. While the decline of the Western Roman Empire was caused by regular migration and expansion of Germanic tribes, the Eastern Roman Empire enjoyed a period of peace and flourished thereon.