Which of the two persons shown in figure (9-Q 1) is more likely to fait down? Which external force is responsible for his falling down?

The person on the right side in the above picture is more likely to fall. The force responsible for his falling is the same force which is responsible for the acceleration of the vehicle that they are sitting in.

Explanation:

From the frame of reference of the vehicle, the two persons experience a pseudo force in the backward direction. While the person on the left does not fall as he gets support from the wall of the vehicle behind his back, the person on the right does not have any support and therefore, is more likely to fall from his seat.