In the 1800s, britain and the united states had laws against unionizing mainly because they believed unions

would cause a lot of violence.

would be bad for the economy.

would brainwash their workers.

would reduce the work hours

Britain and the US believed that Unions would be bad for the economy  - this is the correct answer.

Actually, the answer that Unions would work towards reducing working hours is also correct, but it's not the whole reason: and it's also a part of why they believed Union would be bad for the economy. (so the answer "would be bad for the economy" already encompasses the answer of "would reduce the work hours ")