The correct answer is A) militarism, C) mutual defense alliances, D) mutual offense alliances, E) Imperialism, and F) an assassination.
Some of the underlying reasons that World War 1 began were militarism, mutual defense alliances, mutual offense alliances, Imperialism, and an assassination.
All of those were reasons that began World War 1. But the incident that sparked the fire and provoked the beginning of the war was the assassination of Archduke Franz Ferdinand and his wife in Sarajevo. After this incident, The Austria-Hungary government declared war on Serbia and World War 1 begun.