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

Mass production led to a decrease in the demand for skilled workers. The reason for this was that craftsmen no longer needed to get a boy of a young age who would stay with them for many year where they would teach him the craft they were good at. All of this was changed by machines which were more efficient, faster didn'ttire, and required minimal maintenance compared to a human being. For this reason, the demand for skilled workers decreased. 

I'll go with that