Final answer:
The area of the Austria-Hungary Empire that was ethnically similar to Serbia was Bosnia. It became a focal point for Serbian nationalists and a contentious region due to competing ambitions within the Austro-Hungarian and Ottoman empires.
Step-by-step explanation:
The area of the Austria-Hungary Empire that was ethnically similar to Serbia was Bosnia. Before World War I, Bosnia was controlled by Austria-Hungary and was home to many Serbs, alongside other ethnic groups. This region became the subject of dispute due to Serbian ambitions of creating a greater Serbian nation and uniting the South Slavic peoples.
However, Austria-Hungary, wary of Serbian nationalism and its claims over Bosnia, opposed this movement, leading to rising tensions in the Balkans.
The concept of pan-Slavic nationalism was a critical factor during this period, with Russia also expressing a strong interest in the fate of Slavic nations in the Balkans, leading to international complications.
Later in history, the region experienced further ethnic strain during the violent disintegration of Yugoslavia in the early 1990s, where Bosnia and Herzegovina emerged as an independent state mired in a horrific civil war encompassing various ethnic groups such as the Serbs, Croats, and Bosniaks.