Answer: most likely the assassination of Archduke Franz Ferdinand from Austria
Explanation: The assassination of Archduke Franz Ferdinand made Austria understandably ANNOYED and they went to war with Serbia which most likely had killed the duke.
Answer: The assassination of Archduke Franz Ferdinand, heir to Austria-Hungary.
Explanation: In my opinion, the most important cause of WW1 was the assassination of Archduke Franz Ferdinand, heir to Austria-Hungary. His assassination was the last straw, pushing the Allied and Central Powers to war.
