Correct answer: Bosporus strait. The city of Byzantium was located strategically on the Bosporus, the narrow strait between the Black Sea and the Sea of Marmara.
The city of Byzantium (now Istanbul) achieved its importance due to its control over the Bosporus, and the Bosporan kingdom, greatest state in the northern part of the Black Sea. It also took on particular importance since it allowed Greece to take control of the "Grain Route".