After the Civil War, the United States went saw a major transformation, in which it transformed from being an agricultural society to an industrial one.
Many factors like the massive supplies of raw materials (like timber, iron ore, oil, and others), the development of new technology that made the tasks easier and faster, an important workforce constantly growing by the flow of immigrants, and a new breed of ruthless business leaders were fundamental into the major growth that manufacturing in that time and the following years.
Therefore, the correct answer is Manufacturing.