When a bus stops on a four lane road, who is required to stop?

Answer: B — All traffic.

When a bus stops on a four-lane road, all traffic is required to stop. This rule is in place to ensure the safety of children and other passengers getting on or off the bus. The rationale behind this requirement is that the presence of a stop signal or flashing lights on the bus indicates that it is loading or unloading passengers, and it's crucial for all vehicles to halt in order to prevent any accidents. This regulation applies regardless of the number of lanes in the roadway and includes vehicles traveling in both directions. The design of the road does not permit exceptions for certain lanes or directions; rather, it emphasizes the importance of safety for all road users whenever a bus is engaged in picking up or dropping off children. Therefore, buses act as a clear signal for all motorists to be vigilant and stop accordingly.