Option C, that He went undercover as a worker in a meat-packing plant is the right answer.
Upton Sinclair's (Muckraking Journalist) Socialist Policy urged him to write a Novel called “The Jungle” in 1906, which presented the harsh conditions and abused lives of the immigrants, particularly in the Meatpacking industry of the United States. However, To achieve his goal of presenting this hard condition of the workers he went undercover. Therefore, He used the account of His own investigation to describe the social and economic injustice in the Novel. The Promotion of Socialism in the United States was his main motif while writing the Novel.
The most significant impact of the book can be seen in terms of the “ Pure Food and Drug Act” passed by the government a few months later.
I got this from another answer, but i hope it helped!!