The correct answer is D) by buying up competitors.
Robber barons created monopolies in the United States in the late 1800s by buying up competitors.
Financers, industrialists, and bankers were very greed people in the United States during the 19th century. They became very powerful and influential in the US government. People called them "Robber barons" in a pejorative way because they formed monopolies using trusts and bribing corrupt government officials. They exploited workers, conducted questionable business activities and bought other companies to avoid competition.