What must drivers do when they see a school bus with its stop sign extended?

Answer: B — Stop completely until the sign is retracted

When drivers encounter a school bus with its stop sign extended, they are legally required to come to a complete stop until the stop sign is retracted. This rule is in place to ensure the safety of children boarding and disembarking from the bus. The extended stop sign indicates that children may be crossing the road, and stopping allows them a safe path to do so. Failing to stop can result in severe penalties and significantly increases the risk of accidents involving children. This requirement is crucial for maintaining safety in school zones and during school transportation hours, underscoring the importance of vigilance around school buses.