Answer: Nationalism caused many Serbs living in Austria-Hungary to demand their independence.
Explanation: Tensions increased between Austria-Hungary and Serbia because AH intended to conquer Serbian territories out of fear of what would become the new construction of Serbia.
What is nationalism?
Nationalism is denoted as the advocacy for independence of a region from a higher ruling location. This contributed to the tensions between Serbia and AH because WWI was already inevitable as it was. Additionally, A-H attempted to conquer territories in Serbia so that they would not be able to enter a new era. Because of this, Serbs were caused to demand independence from Austria-Hungary.