Final answer:
The letter from Upton Sinclair to President Theodore Roosevelt discussed the conditions in the Chicago meat-packing industry and the subsequent actions taken by Roosevelt to regulate the industry.
Step-by-step explanation:
The letter from Upton Sinclair to President Theodore Roosevelt discussed the conditions in the Chicago meat-packing industry and how Sinclair's book, The Jungle, had exposed the unsanitary practices of the industry. Roosevelt, after reading Sinclair's book and the subsequent investigation, took action by appointing a commission to investigate the industry and pushed for the passing of the Meat Inspection Act and the Pure Food and Drug Act in 1906. These acts aimed to regulate the meatpacking industry, ensure cleanliness, and prohibit the sale of contaminated foods.