Which war broke out in 1877?

The Russo-Turkish War is the correct answer because it was a significant conflict that began in April 1877 and lasted until 1878. This war was primarily fought between the Russian Empire and the Ottoman Empire. The underlying causes included the nationalistic movements among the Balkan peoples and the decline of Ottoman control in that region. Russia aimed to protect the Christian populations in the Balkans and expand its influence. The outcome of the war had long-lasting effects on the political landscape of Southeast Europe, leading to the independence of several nation-states and altering the balance of power in the region.

The other wars mentioned do not correspond to the year 1877. The Crimean War occurred from 1853 to 1856, the Anglo-Zulu War took place in 1879, and the Boer War began much later, in 1899. Understanding this context helps clarify why the Russo-Turkish War is the event that accurately fits the timeline in question.