Answer:
Correct answer is Nationalism.
Step-by-step explanation:
Nationalism is a correct answer because people who killed Ferdinand were members of nationalistic society, known as Young Bosnia. Their goal was to free Bosnia and other Slavic countries from the influence of Austria-Hungary.
Imperialism and militarism are not correct answers as they are more connected to the role of great powers in that period, including Austria-Hungary, and are some of the reasons why the war started, but not for the assassination.
The same goes with alliances as they were created much earlier. The most famous ones were Central Powers and Entente.