Final answer:
When reversing a school bus and losing sight of the guide, the correct action is to stop immediately to ensure safety.
Step-by-step explanation:
If you are reversing a school bus and you lose sight of the guide, the correct action to take would be to Stop Immediately. This is a safety precaution to ensure that neither the guide nor any pedestrian or object is at risk of being hit by the bus while it is moving backward. Continually honking the horn or asking a pedestrian for assistance are not appropriate actions, as they could increase the confusion or risk around the bus.