Since we are sitting in the bus, our current system is the bus
If any object outside our current system moves, we might think that our system has started moving
We are given the choices:
Passenger moving to the back of the bus: This passenger is moving in our current system. So, his movement will not make us think that the bus is moving
The bus driver turns to look through the windshield: The driver is part of our system and is moving moving within the system. So, we wont think that the bus is moving
A car in the next lane: This car is outside our current system and if it moves, we might think that our current system (the bus) has started moving
Therefore, the movement of a car in the next lane will make us think that the bus is moving