The following was most responsible for enabling the growth of big business in America during the 19th century:
D. the government's laissez-faire approach to the economy
The word or phrase to fill into the gap is the "Ku Klux Klan", an extreme right organization calling for what they would call a "purification" of the US-American society.
I believe the answer is: iconography
Iconography is considered to be a branch of history. The image that painted from a society often represent the social situations or culture that were popular by the time it's made. Which mean by studying such images, historians could obtain valuable information regarding social events or how people at that time think.
