15) F

I think that graph Z is the easiest one to identify.
There are two options given to us for Z: kinetic energy and weight.
Kinetic energy depends on velocity. As velocity changes, kinetic energy changes.
However, weight remains the same no matter what.
Since Z has a straight horizontal line (there is not change), Z must be weight.

Since the car reaches a terminal speed after a while, Y must be velocity. The graph Y becomes flat after a while.

X could either be resultant force or acceleration. However, the only combination possible is option  F. Therefore, X must resultant force.

So the answer must be F.