Which entity was called in to escort nonunion workers to the mills?

The correct answer reflects the historical context of labor disputes, where state militias were often deployed to maintain order during violent strikes or labor unrest. In many instances, especially during the late 19th and early 20th centuries, when strikes became intense and tensions escalated, state militias were mobilized to protect nonunion workers who sought to enter factories or mills. These nonunion workers, sometimes referred to as "strikebreakers," needed protection from striking workers who were often seeking to uphold their rights and demands. The use of militia forces was a common practice during labor disputes as they served to reinforce the authority of factory owners and the government against organized labor actions.

In contrast, while entities like the National Guard, Federal Bureau of Investigation, and State Police could have roles in maintaining public order, they were not the primary forces specifically called upon to escort nonunion workers in the same way the militia historically was during such labor conflicts. The focus of the question on actions taken during labor unrest distinctly aligns with the role of the militia in that era.