In the early and mid-19th century, industry in the united states expanded greatly in part to a) the discovery of gold in the west. b) the expansion of slavery in the north. c) the success of the emancipation movement. d) increased immigration in industrial areas.

D) Increased immigration in industrial areas.

By 1890, production by factories in American had grown eight times larger than what it had been before 1860.  A large part of the workforce came from immigrants coming to the United States and settling in cities, looking for jobs.  Ireland was one example of a country from which many immigrants came, looking for work in America, after the devastation of famine in Ireland (especially from potato crop failures).  

D--increased immigration in industrial areas. 

The increased immigration through the 19th century allowed for cheap labor which industry needs to operate. Immigrants came from Ireland and Germany initially and then much of Eastern and Southern Europe.