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:

Now, we also know that the work is equal to the change of kinetic energy, that is:

Then:

Therefore, we can conclude from the graph that the object's kinetic energy changes by 18 J and that the asnwer is a.