The best analysis of why the United States entered World War I from the options provided is: The U.S. was angered by German unrestricted submarine warfare.
While the assassination of Archduke Franz Ferdinand and the alliances in Europe contributed to the broader context of the war, it was the specific actions of Germany, particularly its unrestricted submarine warfare that threatened American lives and commerce, that directly influenced the U.S. decision to enter the conflict. This policy led to the sinking of ships with American passengers and cargo, such as the Lusitania, which heightened public outrage and pushed the U.S. government towards war.