Answer:
The right answer is "to protect convoys of merchant ships."
Step-by-step explanation:
As the other person stated, one of the main issues was that Germany persisted engaging in unrestricted marine warfare, attacking what they perceived as threats, mistakenly thinking the ships were carrying mines or munitions. A notable example is the sinking of the USS Lusitania. I also just completed the exam myself, and received full credit with this response.