The most likely cause of the trend shown on the graph, which indicates a significant increase in the number of manufacturing workers in the United States from 1850 to 1900, is:
The growth of industrialization and the development of factories.
This option directly explains the rise in manufacturing jobs due to the industrial revolution, which saw a shift from agrarian economies to industrial economies with the establishment of factories. While increased immigration contributed to the workforce, the primary driving force behind the increase in manufacturing jobs was the broader trends in industrialization.