Which statement describes a reason the united states entered world war i on the allied side?
a. germany's invasion of canada posed a direct threat to the united states.
b. president wilson wanted to take possession of germany's colonies.
c. german u-boats sank unarmed ships carrying american passengers.
d. most americans believed that europe's fight was also their fight.

C. German U-boats sank unarmed ships carrying American passengers.

This was the reason the United States entered World War I on the Allied side.

Germany was at war with Great Britain. But, they were also attacking civilian vessels and even ships that has flags that declared their neutrality. 
The first time German U-boat sank an American vessel, they apologized to the United States and vowed to avoid attacking vessels that are not included in their fight against Great Britain. However, Germany did not stand by their oath. They kept on attacking any vessels found in the British seas.