Which work by Upton Sinclair exposes the harsh conditions of the meatpacking industry?

The correct choice is A, "The Jungle." This novel, published in 1906, is seminal in American literature for its vivid depiction of the unsanitary and inhumane conditions within the meatpacking industry in Chicago. Sinclair's work was intended to promote socialism and reform labor practices, but it is best remembered for its graphic portrayal of the exploitation of immigrant workers and the dangerous practices utilized in food production. The public response to the novel prompted significant changes in food safety regulations, such as the establishment of the Food and Drug Administration (FDA) and the implementation of the Meat Inspection Act, highlighting its powerful impact on American society and reform movements.

The other options, while notable works by Upton Sinclair, do not focus on the meatpacking industry. "Oil!" deals with the oil industry in California; "King Coal" explores the coal mining industry, highlighting the struggles of miners; and "The Flivver King" is a critique of the automobile industry and its effects on labor. Each of these works addresses important social issues but does not delve into the specific concerns over meatpacking that are central to "The Jungle."