The correct answer is C.
A monopoly is a market structure where a single firm serves the whole demand of a specific good or service. It does not face competitors, therefore, such firm has absolute market power to decide the price charged for its products. So, the monopoly is able to charge a higher price than in a perfect competition scenario and will earn much larger profits.
The accumumlation of such vast amounts by the industrial leaders, gave rise to the emergence of the Progressive movement in the US. It took place between the 1880s and the 1920s with the aim of eliminating negative consequences of the industrialization process in the US and monopolies were targeted. Monopolies enriched their owners by preventing competition and they were harmful for consumers because these were forced to buy worse quality products at higher prices. This is the origin of antitrust regulations and of the interference of public powers in the regulation of private businesses.