Option A, they were among the first to publicize immoral, corrupt practices of large industries are the right answer.
All the three Upton Sinclair, Ida Tarbell, and Frank Norris were the muckrakers, who presented Americans with details about corruption in industry and government. While, Upton Sinclair presented the harsh conditions and abused lives of the immigrants, particularly in the Meatpacking industry of the United States. Ida Tarbell wrote a set of articles to expose the corruption in the Standard Oil to end the monopoly. Same was the case with Frank Norris, who wrote about the monopoly of railroads in California.
Your answer is A. Sinclair wrote about the horrible conditions in the Chicago meat packing district, Tarbell's investigating reporting helped break up Standard Oil, and Norris' work includes depictions of suffering caused by corrupt and greedy corporate monopolies.
