D. The city of Istanbul became a major center of the Muslim world.
With the Ottoman's conquest of Constantinople, now called "Istanbul", in 1453, The Ottoman leader Sultan Mehmed II declared the city his new capital, and turned the former Christian city into a major center of Islam, turned the Hagia Sophia (the main Greek cathedral at the time) into a Turkish mosque, as well as many other cathedrals, initiated reconstruction projects for Constantinople and took several actions to re-populate it, among others.
