An advocate of laissez-faire policies during the late 19th and early 20th centuries would have been interested in which of these? protecting farmers promoting industrial growth expanding union membership preventing foreign competition

The correct answer is: "promoting industrial growth".

Laissez-faire supporters believed that the most efficient economic outcomes were reached by the free interactions of economic agents (households, firms and public sector entitities) in the markets, and that external goverment interventionism distorted such outcomes, that maximized the welfare of all the market participants.

Therefore, protecting farmers, expanding union membership and preventing foreign competition cannot be measures supported by advocates of laissez-faire policies because they describe forms of interventionism.

b. sec, foreign relations