T-t which event in april 1945 caused a sudden shift in american leadership in the planning of the united nations? a) the end of world war ii b) the beginning of the red scare c) the death of president roosevelt d) the return of american soldiers from war

It's A, Europe was devastated, Japan had surrended, the US along with the URSS emerged as the superpowers of the time, with the US getting the upper hand in most cases by having a larger area of influence for instance.

B this is related to a phenomenon that first happen after the Russian Revolution in 1917 and then shortly after WWII in 1947. Red Scare is a kind of paranoic fear of people who might be communists or spies to the URSS.

C happened in 1945 but there is no cause-effect justification for the assention of the US in the UN (how come the death of president magically turns the country into the leader of an international organization?)

D is related to A but it is something in the concern of the people directly involved and/or had relatives involved in the battlefield,not the externalk politics.