Answer regarding a cause of tension leading into World War II:
C. Japan began establishing an empire by attacking and conquering its neighbors.
Context/detail:
The conflicts that became part of World War II actually began earlier than the 1939 start of the war in Europe. In Asia, Japan was beginning its imperial conquests before Hitler and Germany began expanding into Czechoslovakia and invading Poland. Japan invaded the Chinese territory of Manchuria in 1931.
The League of Nations condemned Japan in 1933 for the events in Manchuria, but that did little to stop Japan. Japan withdrew from the League of Nations at that time, and by 1937 began further invasions into Chinese territory. Japan's imperial ambitions even reached the point of genocidal acts such as the massacre at Nanking in 1937-1938.
When Japan moved into French Indochina in 1941, the USA froze all Japanese financial assets in the USA and placed an embargo on all oil and gasoline shipments to Japan. The Japanese viewed the embargo as an act of war, and their attack against the US at Pearl Harbor was (from their viewpoint) a response to US trade sanctions against them.
