This question is a good example of the difference between 'speed' and 'velocity'. This one is all 'velocity', and you've absolutely gotta keep the directions straight.
The object started out moving forward at 10 m/s. It ended up moving backward at 2 m/s. What was the change in its velocity ?
The change was 12 m/s backward !
Momentum = (mass) x (velocity).
Change in momentum = (mass) x (change in velocity)
This change in momentum = (0.1 kg) x (12 m/s backward)
This change in momentum = 1.2 m/s backward or 1.2 Ns (B)
