Final answer:
Motorists must stop at least 20 meters behind a school bus with flashing red lights when it is loading or unloading passengers to ensure the safety of children.
Step-by-step explanation:
The question asks for the regulations concerning the distance a motorist must stop behind a school bus that is loading or unloading passengers with red signal lights flashing. The precise distance may change based on the relevant jurisdiction. However, in many places, the rule is that motorists approaching from behind must stop at least 20 meters away from the bus. Stopping this distance allows children to enter and exit the bus safely.