When should the engine of the bus be turned off?

The correct choice emphasizes the safety measures that are essential when loading and unloading students near a school. Turning off the engine during this process minimizes the risk of accidental movement and reduces the chances of harmful fumes affecting students. By shutting off the engine, the driver can also ensure that all attention is focused on the students, facilitating a safe environment as they board or disembark from the bus. This procedure is particularly important within 100 feet of a school site, where children are often present and may be unpredictable.

Other options address different scenarios, such as parking or stopping at traffic lights, but these do not necessarily focus on the specific safety protocols related to the presence of students.