What is the purpose of a bus's safety interlock system?

Answer: B — To prevent the bus from being put in gear when doors are open

The primary purpose of a bus's safety interlock system is to ensure safe operation by preventing the bus from being put into gear when the doors are open. This feature is critical for the safety of passengers, especially children, as it prevents the vehicle from moving while the doors are accessible, minimizing the risk of accidents during boarding and disembarking. By requiring the doors to be closed before the bus can be shifted into gear, this system adds an essential layer of protection that is crucial in a school bus environment where safety is the top priority. The other options relate to functions that do not pertain to the safety interlock system. For example, running at higher speeds, adjusting the engine's fuel supply, or activating emergency brakes are not relevant to the specific safety mechanism designed to control door and gear engagement.