The correct answer is D, as the German unrestricted submarine warfare was a factor in the United States entrance in World War I.
In April 1917 the United States declared war on the Central Powers, which gave the war a global character. However, its effects would not be felt until 1918. The sinking by a German submarine of the RMS Lusitania on May 7, 1915, where 128 Americans died had generated an anti-German popular feeling among Americans, but it was the German announcement of a submarine war "without restrictions" together with the confirmation of the authenticity of the Zimmermann telegram which caused a flow of anti-German sentiments in the United States, and was the trigger for its official entry into war on the side of the allies.
