The correct answer to this open question is the following.
Although there are no options attached, we can say the following.
The opportunities that the Germans exploited to gain its few U-boat successes and force the United States and allies to adjust tactics were that they successfully identified the patrolling time of the Allied ships in the zone. This gave the U-boats a clear advantage to know when to attack the cargo or commercial ships that were not protected by the Allies' warships on the shores of England. That is why German U-boats had success attacking allies' targets, as was the case of the Lusitania ship.
So we can say that the major success of the German U-boat in World War I was that they sunk neutral commercial and cargo ships in an attempt to cut off supplies to Britain and weakened its positions in the sea.
