Final answer:
Upton Sinclair was the vegetarian activist who exposed the horrific conditions in the meat packing plants of Chicago. His novel, The Jungle, revealed unsanitary practices in the beef packing industry and led to reforms in food regulations.
Step-by-step explanation:
The vegetarian activist who exposed the horrific conditions in the meat packing plants of Chicago was Upton Sinclair. Sinclair's novel The Jungle revealed the unsanitary practices of the beef packing industry, inspiring readers to challenge the capitalist system and leading to the passing of the Meat Inspection Act and the Pure Food and Drug Act in 1906.