Begin the answer by stating that the outbreak of World War I was caused by a collaboration of multiple factors.
Then, elaborate on the reasons that we learnt
Alliances
Violent nationalism
German aggression
Germany-France rivalry
Balkan conflicts
Assassination of Austrian emperor
Solution:
The first world war broke out due to the cumulation of multiple factors.
Alliances in Europe: 20th century Europe was divided into two camps drawn from competition and nationalism. Britain, France, and Russia formed Triple Entente ( allies) while Germany, Austria, and Italy formed Triple Alliance.
Violent Nationalism: Military competition among the countries led to violent forms of nationalism. England’s jingoism, France’s chauvinism, and Germany’s Kultur were militant forms of nationalism that promoted hatred for each other.
German aggression: Kaiser Wilhelm II, the emperor of Germany, was ruthlessly assertive and aggressive. The rapid expansion of the navy was perceived as a threat to Britain. This heightened the tension between the two European powers
France-Germany rivalry: The countries had an old rivalry. Upon that, Germany interfered in the Morocco question, which resulted in increased tension between the two.
Balkan conflicts: Austria’s annexation of Bosnia and Herzegovina aroused opposition from Serbia. As Germany supported Austria, it promised to go against Russia if Austria invaded Serbia and Russia tried to help them.
Assassination of Archduke Franz Ferdinand: The killing of the titled Austrian emperor provided a solid reason for Austria to invade Serbia. The Austria-Serbia conflict prompted Germany to declare war against Russia. Because of the Russian-Franco alliance, Germany had to declare war against France too. Soon, Britain joined the war.