Question

what would cause a backpack sitting on the floor of the school bus to suddenly fly forward?
the bus makes a hard turn to the left.
the bus is going backward and suddenly stops.
the bus is going forward and suddenly stops.
the bus makes a hard turn to the right.

Explanation:

According to Newton's first - law of motion (inertia), an object in motion tends to stay in motion. When the bus is going backward and suddenly stops, the backpack, which was moving backward with the bus, will continue to move in its original direction (forward relative to the now - stopped bus) due to inertia.

Answer:

The bus is going backward and suddenly stops.