(a) The forces acting on the helium balloon is the upward force exerted by the balloon on the roof of the wagon and the normal force exerted by the roof of the wagon on the balloon. The balance between the forces account for the vertical equilibrium of the balloon.Similarly, the beach ball is in vertical equilibrium because of the balance between the weight of the ball and the normal force from the bed on the ball.
When a car turns the corner at constant speed, centrifugal force acts on the balloon, and the ball. This force accelerates the balloon and the ball towards the sides of the car away from the axis of rotation.
(b) Assume that the car was initially moving with a uniform speed, and then started to decelerate by applying the brake. Due to inertia of the objects, the balloon on the roof and the ball on the bed would maintain its motion and keep on moving with same speed in the same direction, while the car would come to rest.For the observer in car, a force would appear to act on the balloon and the ball, which has accelerated them to move from their position towards the front of the car.