The Byzantine Empire became a regional center of trade because:

b: it  controlled the city of constantinople

The answer is B: it controlled the city of Constantinople.

Explanation:

Constantinople lays on the Bosporous straight which separates Europe and Asia. The Bosporous straight also linked the Mediterranean Sea and the Black Sea-important for trade. This allowed the city to control all trade routes between Europe and Asia. So controlling Constantinople meant you controlled trade.