Final answer:
Pennsylvania was the location of the oil fields supplying Rockefeller's refinaries, which was at the heart of the early American oil industry.
Step-by-step explanation:
Most of the oil fields supplying Rockefeller's refineries were located in C) Pennsylvania. During the early days of the oil industry, Pennsylvania was at the forefront. Edwin Drake's famous well near Titusville, Pennsylvania, was drilled in 1859 and for a time, the Bradford Field in Pennsylvania produced a significant portion of the world's oil supply. Rockefeller, through his company Standard Oil, capitalized on the abundance of oil in Pennsylvania to become a leader in the oil refining industry. The state was pivotal in the establishment and growth of the American oil industry before oil discoveries spread to places like Texas and Alaska in later years.