Which of the following hazardous materials are prohibited from being transported by bus?

When considering prohibited hazardous materials for transportation by bus, it's important to recognize the specific regulations that apply. The correct answer identifies that transporting more than 100 lbs of solid Class 6 poison is not allowed. This is primarily because Class 6 poisons are toxic substances that can pose significant health risks, not only to passengers but also to emergency responders and the general public in the event of an incident.

In the context of bus transportation, safety is paramount. Buses typically carry numerous passengers, making the transportation of highly toxic materials particularly dangerous. The regulations are designed to minimize the potential for severe outcomes in the event of an accident or leak, which is why limits are strictly enforced regarding hazardous materials.

While the other hazardous materials listed, such as Class 3 flammable liquids, Class 1 explosives, and Class 4 flammable solids, are indeed hazardous, they are categorized under different levels of restrictions based on their properties. For instance, while Class 1 explosives are generally prohibited, there are specific exemptions and means to transport such materials under strict guidelines. Hence, the focus on solid Class 6 poisons, especially in quantities exceeding a certain limit, reflects heightened concerns for passenger safety and public health in a bus environment.