Answer:
The answer is: D. Investigative journalists whose work sought to bring about social reform and relief for the lower classes.
Step-by-step explanation:
According to the Cambridge Dictionary, muckraker is a person, especially one in a news organization, who tries to find out unpleasant information about people or organizations in order to make it public.
Upton Sinclair was an American novelist, essayist, playwright, and short-story writer, who wrote "The Jungle" a 1906 novel . It was written to portray the harsh conditions and exploited lives of immigrants in the United States in Chicago and similar industrialized cities.