The correct answer is C) He went undercover as a worker in a meat-packing plant.
The other options of the question were A) He hired investigators to spy on activities in a meat-packing plant. B) He spent two years doing research in libraries all over America. D) He interviewed experts in the meat-packing industry.
The statement that best describes the main way that Union Sinclair gathered research for the jungle is "He went undercover as a worker in a meat-packing plant."
When American journalist Uptown Sinclair wrote the book "The Jungle" in 1906, the publication became a success because in the book he described the harsh and unhealthy conditions of the factories and the way people worked there. He referred to the meat packing industry and the lack of sanitary regulations. His investigation helped to change some regulations in the United States.