Which of the following was NOT regulated by the Interstate Commerce Commission?

Answer: C — Types of fuel used by trucks

The correct choice highlights that the types of fuel used by trucks were not regulated by the Interstate Commerce Commission (ICC). The ICC was primarily established to regulate the railroad industry and later expanded its authority to other forms of transportation, including trucking. However, it focused on ensuring fair rates, preventing discrimination, and overseeing operational aspects such as market entry and safety. While the ICC did influence areas related to trucking—such as the markets trucks could serve and the number of hours a truck driver could work—fuel type standards and regulations regarding the types of fuel were typically governed by environmental and energy agencies rather than the ICC. This distinction is crucial in understanding the scope and limitations of the ICC's regulatory authority. Thus, the types of fuel used by trucks fall outside the ICC's purview, making it the correct answer to the question.