Who won the Russo-Turkish War and threatened to take a key city?

The outcome of the Russo-Turkish War, which lasted from 1877 to 1878, saw Russia emerge victorious. The war was fought primarily over the independence of Balkan nations from Ottoman rule, and it significantly shifted the balance of power in the region. Russia’s victory allowed it to exert influence over the territories that were once part of the Ottoman Empire, including a pathway to the previously contested city of Constantinople (Istanbul) and other key regions in the Balkans.

This victory for Russia threatened the Ottoman Empire's hold on its territories, leading to increased nationalist movements within the empire and a reconfiguration of borders, particularly with the creation of the Principality of Bulgaria and other states. Russian military success was pivotal in asserting its dominance in Eastern Europe and played a crucial role in the geopolitical landscape of the time.

The other choices do not align with the outcome of the war, as Turkey (the Ottoman Empire) was defeated, Bulgaria was a beneficiary of Russia's victory rather than the aggressor, and Greece, while having participated in previous conflicts against the Ottomans, was not the dominant force in this particular war.