The correct answer is "A".
The Schlieffen Plan was the name given to the German war plans to invade France and Belgium during the First World War. The plan was put into effect with the invasion of France on August 4, 1914.
Schlieffen, who was chief of the General Staff of the German Army at the time, estimated that if a rapid attack was made on France, they would not be able to support Russia when the Germans decided to attack them, therefore avoiding a potential conflict on two fronts.