In an outline, one of these entries is a main topic and three are subtopics. which is the main topic?

Assuming that this is referring to the same list of options that was posted before with this question, the correct response would be "Darwin's methods," since this is the "umbrella" under which the other topics lie. 

john maynard keynes developed new economic theories in response to the stock market crash of 1929. great depression of the 1930s. first and second world wars. economic boom of the 1950s.