How do you prepare to leave once all students are accounted for?

Answer: D — Turn off the flashing red lights and turn on the left turn signal

To prepare for leaving once all students are accounted for, turning off the flashing red lights and activating the left turn signal is the correct action. Flashing red lights indicate that students are loading or unloading, so they must be turned off to signal that it is safe to leave. Activating the left turn signal communicates your intention to turn left, which is essential for ensuring the safety of other drivers and pedestrians around you. This procedure indicates that you have ensured all students are safely in the bus and that it's time to proceed, thereby maintaining safety protocols, which is a vital aspect of operating a school bus. The other options do not sufficiently cover the necessary steps to ensure a safe departure from a stop, such as signaling your intentions to other road users.