Which of the following individuals is NOT allowed to stand on a school bus in motion?

The choice indicating that any passenger is not allowed to stand on a school bus in motion is correct because safety regulations specifically prohibit any passenger from standing while the bus is moving. This regulation is in place to protect passengers from the risks associated with sudden stops or accidents, as standing can lead to serious injuries or loss of balance.

The other individuals listed, such as an adult acting upon the driver's request, a trainer, and a mechanic, may have specific circumstances where temporary standing might be permitted under controlled conditions or for safety training purposes. However, these situations come with strict guidelines and typically involve direct instruction or oversight from the driver, ensuring that the overall safety of all passengers is maintained. In contrast, the general rule is that all passengers on a school bus, particularly children, must remain seated when the bus is in motion to ensure their safety.