Displacement is represented by the area under the velocity curve. So we just calculate the area of the triangle we see i.e. 1/2 * 4 * 8= 16. But the particle starts at 2 so we have 16+2=18. Thus D
To see the "displacement under the curve" explicitly you can observe
integrating wrt t
integrating wrt t
You can see from the graph that maximum displacement must occur when velocity changes sign (direction) at t=4, or...