We know that in a force-position graph the area under the graph is equal to the work done by the force.
In this case the area under the graph is a triangle with base 3 and height 12, then the work done is:
[tex]\begin{gathered} W=\frac{1}{2}(3)(12) \\ W=18 \end{gathered}[/tex]
Now, we also know that the work is equal to the change of kinetic energy, that is:
[tex]\Delta K=W[/tex]
Then:
[tex]\Delta K=18[/tex]
Therefore, we can conclude from the graph that the object's kinetic energy changes by 18 J and that the asnwer is a.