The answer is A. U-boat warfare was the German response to the British blockade.
Explanation:
As Germany had only limited access to the ocean and since it was primarily a land based empire, naturally they did not possess a strong navy. However, the British Royal navy was the largest and the most powerful navy at the time. Because of this, Royal navy was able to blockade the German ports and the ports of German allies and prevented them from resources and other supplies from the sea.
This is why the Germans developed underwater U boat warfare to defeat the royal navy and their blockades.
