In the 1800s, mass production led to?
an upheaval in the workers’ unions
a deep recession after the civil war.
a decrease in the demand for skilled workers.
a decrease in child labor.

The correct answer is: "a decrease in the demand for skilled labor".

The sucess of the Industrial revolution and the development of the factory production model increased the demand for unskilled workers. Many of those came from overcrowded unproductive agricultural works in the countryside, and had moved to cities seeking for a better way of life.

Production processes were divided in many simple steps therefore, learning how to perform efficiently one of those steps, did not require very high qualifications.

c or d both can be the answer