Which group suffered most from the formation of a monopoly?

unions

politicians

small businesses

large corporations

The correct answer is "small businesses"

In the 19th century, monopolies harmed many, especially small businesses. For example, small farmers complained that monopolistic railroads engaged in price gouging for shipping their produce.

Monopolies stopped businesses from forming and closed the ones who where working, they bought competitors, upper-priced them, forced costumers to sign contracts.

At that time the US government believed strongly on laissez-fair capitalism, that meant that the system of capitalism would be responsible to regulate itself.