The most likely entry in the last balloon for reasons for U.S. entry in WWI is Unrestricted submarine warfare.
What is Unrestricted submarine warfare?
This was a policy that the Germans used in the first World War whereby they targeted all ships headed for Allied ports including civilian ships without giving them warning that they would be attacked.
This meant that American civilians headed to Britain or France were in danger of being casualties from a German attack and so the United States could not stand by any longer.
In conclusion, option C is correct.
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