Final answer:
The secret Serbian nationalist group that opposed Austria's annexation of Bosnia was the Black Hand, which played a significant role in World War I.
Step-by-step explanation:
The secret Serbian nationalist group that opposed Austria's annexation of Bosnia was called the Black Hand. It was formed in 1911 with the goal of uniting all Serbian territories and achieving independence. The group was responsible for the assassination of Archduke Franz Ferdinand of Austria-Hungary, an event that triggered the start of World War I.
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