Gravitational potential energy of the car is given by formula

here m = mass of the car
g = gravitational field intensity
h = height of the car from reference point
so here from above formula we can say that if height of the car from reference is zero the gravitational potential energy will be zero
So here we generally took the reference point to be at bottom on the ground
so here also the gravitational potential energy will be zero at the ground
so correct answer will be
C)at the bottom of the hill