Answer:
Correct answer is Triple Alliance
.
Explanation:
First option is not correct as this is a war strategy used in many wars including WW1.
Schlieffen Plan is a German war plan on the western front.
Triple Alliance is a military alliance which Bulgaria later joined during WW1 that defended its interest.
Kaiser Wilhelm was a German Emperor at the time.
Gavrilo Princip killed Franz Ferdinand.
Triple Entente fought against Bulgaria.
