B is the answer because the area between the velocity time graph and the x-axis is the distance traveled. If you look at the velocity time graph from 0-2, seconds you will see an area of 40m. Only choice B matches that at 2 seconds, the distance traveled is 40m. You can go on to confirm the remaining trip matches that of B by calculating the 2-4 seconds of the velocity time graph area between the x-axis and graph as -40m (negative because area is under the x-axis). Again, it matches graph B, showing that the total distance traveled is 0 for the entire trip.
