Which of the following ideas is a major element of trickle down economics?
rich countries should provide financial aid to poor countries.
corporations should be taxed heavily to pay for social welfare programs.
high levels of military spending are harmful to the economy.
reducing taxes on wealthy americans will also benefit the poor.

Reducing taxes on wealthy Americans will also benefit the poor.

The Trickle Down Economics is believed to an idea that when those earners who have high income earn more, then this would also result that everyone in the economy can benefit from this. So based on the options above, the one that shows the major element of this type of economics is option D. 

The Battle of Britain

Explanation:

Battle of Britain, during World War II, the successful defense of Great Britain against unremitting and destructive air raids conducted by the German air force (Luftwaffe) from July through September 1940, after the fall of France.

Source: https://www.britannica.com/event/Battle-of-Britain-European-history-1940