Final answer:
Trucks are the type of motor vehicles that must stop at all railroad grade crossings outside of business or residential districts, primarily due to their size and potential cargo.
Step-by-step explanation:
The vehicles that must stop at all railroad grade crossings outside a business or residential district include certain large or heavy motor vehicles, such as vehicles carrying passengers for hire (like buses), vehicles carrying hazardous materials requiring placarding, and school buses, among others. The options given were motorcycles, bicycles, trucks, and all of the above.
Of these options, the correct answer is usually trucks, as they often fall under the types of vehicles required to stop due to their potential cargo and weight. However, specific regulations can vary by locality, so for a comprehensive answer, one should refer to local laws or the commercial driver's handbook. Motorcycles and bicycles do not typically fall under these stopping requirements unless local law specifies otherwise.