When is it acceptable for a school bus driver to use a cellular phone?

Answer: B — When safely parked or in emergencies

The correct answer highlights that school bus drivers should only use a cellular phone when they are safely parked or in emergencies. This is vital for ensuring the safety of children on board and other road users. Using a phone while driving, even cautiously, can be a significant distraction and increases the risk of accidents. When a driver is parked, they can devote their full attention to making calls or handling messages without compromising the safety of their passengers. In emergency situations, a quick call could be necessary to ensure assistance, but it should always be done with the understanding that the safety of the bus and its occupants takes precedence. The other options suggest scenarios where phone use might distract a driver from their primary responsibility of safely navigating the bus, which is why they are less appropriate.