Which of these was the MOST direct cause of World War I?

The correct answer is: the assassination of Archduke of Austria-Hungary - Franz Ferdinand.

Explanation:

Franz Ferdinand, the Archduke of Austria-Hungary and his wife Sophie were assassinated by Gavrilo Princip in 1914, in Sarajevo.

The political motives for his assassination were the desire to separate the Austro-Hungarian province of Bosnia and Herzegovina and its unification with Great Serbia. The assassination of Ferdinand led directly to World War I when Austria-Hungary gave an ultimatum to Serbia, which was partially denied. Austria-Hungary declared war on Serbia, which triggered a series of war declarations between other European countries.