Which of these terms best describes the official foreign policy of the united states at the start of world war ii in 1939?

It was isolationist.  The Americans were in the midst of rebuilding their economy that was now improving after the Great Depression and the effects of World War II.  Many did not want to go to another war but after witnessing the atrocities committed by the Axis powers in Europe and Asia, they realize that it would only be a matter of time before they would be involved in the war.  When the Japanese bombed Pearl Harbor the U.S. officially joined the war on the side of the allies.

our current president donald trump