In which situation is it acceptable for a school bus driver to use a cellphone?

Using a cellphone while driving a school bus is heavily regulated for safety reasons. The correct answer emphasizes that a school bus driver may only use a cellphone when the bus is at a complete stop. This is crucial because it ensures the driver's full attention is on the road and the passengers during operation. A complete stop means that the vehicle is not in motion, allowing the driver to momentarily divert their attention to the phone without compromising safety.

In contrast, the first option implies that communication with dispatch is a valid reason to use a cellphone while driving, which is not permitted as it distracts from the primary duty of operating the bus safely. The third option suggests that using a cellphone during breaks is acceptable; while this is true outside of driving duties, it doesn’t address the situation of operating the bus. The final option about responding to text messages also fails to recognize the importance of maintaining focus while driving, as any form of texting is inherently distracting, regardless of the bus's motion status. Thus, using a cellphone is limited to when the bus is completely stopped to prioritize the safety of the passengers and the overall operation.