Final answer:
School buses carrying children, buses carrying passengers, and vehicles carrying explosive substances must stop at uncontrolled main railway crossings; hence, the correct answer is d. a. and c. are correct.
Step-by-step explanation:
Among the vehicles required to stop at uncontrolled main railway crossings are school buses carrying children, as well as other buses carrying passengers. Additionally, any vehicle carrying explosive substances is required to come to a complete stop at these crossings to ensure safety. Therefore, the correct answer to which vehicles are required to stop at uncontrolled main railway crossings is d. a. and c. are correct. The vehicles that are required to stop at uncontrolled main railway crossings are:
School buses carrying children and all other buses carrying passengers: These vehicles must always stop at railway crossings to ensure the safety of the passengers on board.
Vehicles carrying explosive substances: These vehicles must also stop at railway crossings to prevent any accidents or mishaps.
Therefore, the correct answer is d. a. and c. are correct.