Final answer:
John D. Rockefeller expanded his investments through control over the transport and distribution aspects of the oil industry, utilizing aggressive business practices to reduce competition and establish a monopoly with Standard Oil.
Step-by-step explanation:
After consolidating his position in the oil industry through both horizontal and vertical integration, John D. Rockefeller continued to expand his investments by gaining control over various aspects of the oil production and distribution process. These strategies included the acquisition of pipeline and transportation companies, thereby strengthening Standard Oil's dominance. The goal was to control every stage of the product's lifecycle, from raw material to consumer delivery, which ultimately allowed Rockefeller to effectively lower kerosene prices and establish a monopoly over the oil industry.
Rockefeller used this control to drive competitors out of the market and grow Standard Oil into a conglomerate through aggressive business tactics. He forged secret deals with suppliers and railroads, and the infrastructure for the transportation of oil was an essential focus of his business expansion, not investing in unrelated industries like the automobile, steel, electric power, or agriculture sectors as suggested by the multiple-choice options in the question.