How did imperialism, nationalism, and militarism contribute to making this a world war?
Imperialism is when countries increase their power and wealth by controlling other countries and using their natural resources. Before World War I, many countries in Africa and Asia were under control of European countries. Raw materials those countries could provide were the main reason of competition and desire of greater empires for control over them.
Militarism is when countries largely invest in military - development of new weapons and training of soldiers. Before World War I Germany and Great Britain both invested greatly in navy. In Germany and Russia military establishment had a huge impact on public policy. With developed new technologies and weapons war was inevitable.
Nationalism is when a country’s politics is that maintaining self-control is the most important. This led to desire of Slavic people of Bosnia to no longer be a part of Austria Hungarian Empire but to be a part of Serbia - another Slavic country. It led directly to war.