The correct answer is A. The Zimmerman Telegram
Step-by-step explanation
The Zimmermann Telegram was a telegram sent by Arthur Zimmermann (German Foreign Affairs Minister) to the German ambassador to Mexico with the objective of delegating him the task of proposing an alliance to the Mexican government against the United States. This plan was intercepted by the British spying services, who shared it incognitably with the United States government, accelerating their entry into the war. So, the correct answer is A. Zimmerman Telegram.