Free body diagram (a) is the correct free body diagram.
Work Step by Step
Free body diagram (a) is the correct free body diagram. At the bottom of the hill, the tangential acceleration is zero. The car is moving in circular motion so the acceleration points toward the center of the circle, in this case, straight up. Therefore, the net force must be directed straight up. Free body diagram (a) is the only free body diagram with a net force which points straight up, as the normal force from the road is greater in magnitude than the car's weight.