Which of the four reasons were the most significant for American involvement in WWI?

America joined the war for four main reasons:

1.  German unrestricted submarine warfare was sinking American neutral vessels that were trading the Britain and France.  America almost joined the war over the loss of 128 American lives when a German U-boat sank the Lusitania in 1915.  Germany was forced to back away from its policy of sinking all vessels in British waters, but by 1917 Germany resumed this practice in order to try to win...

America joined the war for four main reasons:

1.  German unrestricted submarine warfare was sinking American neutral vessels that were trading the Britain and France.  America almost joined the war over the loss of 128 American lives when a German U-boat sank the Lusitania in 1915.  Germany was forced to back away from its policy of sinking all vessels in British waters, but by 1917 Germany resumed this practice in order to try to win the war.  

2. The Zimmerman note, which was a secret deal between the German Empire and Mexico that would mean Mexico declaring war on America long enough to distract the United States from the larger war in Europe.  Germany pledged support, and after the war Mexico would receive the lands lost during the Mexican War fought against America from 1846-1848.  While the German government in Berlin was probably never aware of Ambassador Zimmerman's plan to make Mexico a nominal Central Power ally, British intelligence delivered the note to Washington, and furthered the idea that Germany could not be trusted.  

3. American war loans to the Allies would be in danger of not being paid back if the Allies lost the war.  Millions of dollars of aid in the form of loans and weapons sales were going to the Allied Powers well before 1917.  Germany appeared close to victory in late 1916 and early 1917, with Russia reeling in the East and the French military having mutinies on the Western Front.  Something had to be done to ensure that America's financiers received their money back with interest.  

4. Allied propaganda ensured that only one side of the story of the war was told.  Early in the war, the British cut the German telegraph cable to America.  From that point, the British ensured that only Allied war news was spread.  When Germany invaded Belgium, Americans heard of Germans attacking and enslaving civilians, when in reality this practice was not as widespread as reported.