The robber Barons became rich by building monopolies. Option D is correct.
"Robber baron" is a derogatory metaphor of social criticism which was first refered to some late 19th-century American businessmen who were accused of using unscrupulous methods in their attempt to become wealthy, or expand their wealth.
The metaphor became popular as early as February 9, 1859, when The New York Times employed it to describe the business practices of Cornelius Vanderbilt.
