What must you do as driver of a school bus before leaving a bus stop?

The combination you selected is correct because it addresses essential safety precautions that a school bus driver must take before leaving a bus stop.

1. Ensuring all doors are properly closed is critical to prevent any child from accidentally falling out or trying to exit the bus after it has started moving.
2. Checking that no child is in front of the bus is vital as this area is a blind spot for the driver. It's important to confirm that children are a safe distance away before proceeding.
3. Turning off the hazard warning lights indicates to other road users that the bus is leaving the stop, promoting safer traffic flow.

Together, these steps help ensure the safety of children and other road users, complying with German traffic regulations aimed at minimizing the risk of accidents involving school buses.