Which one of the following bodies has only potential energy?
a. a ball rolling downhill
b. a cat sitting on the fence
c. a dog running after a car
d. a stone falling from a cliff

Potential energy is energy of position, while kinetic energy is energy
of motion.  The same object may have some of both at the same time. 
But in order to have kinetic energy, the object must be moving.  If it
isn't moving, then it can only have potential energy.

The only object listed among the answer choices that's not moving is
the cat sitting on the fence.  The other choices involve rolling, running,
and falling.