The correct answer is C) German submarine attacks.
Tensions grew between the United States and Germany with the German submarine attacks.
We are referring to the Battle of the Atlantic. German navy started to attack US and allied ships in World War II. Germany attacked them using their modern submarines that shot cargo ships or battleships. Both sides, Allies and Axis powers, wanted to control navigation in the Atlantic. Navigation through the Atlantic Ocean was vital for the Allies to transport supplies to the war front, but the German navy wanted to cut those supplies.