The correct answer is C) ethnic groups withing the empire resented their lack of self-determination.
The statement that most accurately explains how the union of Austria and Hungary contributed to the outbreak of World War I is ethnic groups withing the empire resented their lack of self-determination.
The Austria-Hungary Empire was a constitutional monarchy that governed these territories from 1867 to 1918, the end of World War 1. Both countries signed the Austra-Hungary Compromise, on March 30, 1867. The agreement included the region known as Croatia-Slavonia territory. Each monarchy oversaw the domestic issues of each empire, and joint together in foreign affairs and military issues.
The other options of the question were A) the powerful economy of Austria-Hungary dominated Europe and upset neighboring countries, B) Germany feared the power of the Austria-Hungary empire and began to militarize, and D) the union eventually created tension between Austria and Hungary that led to war.