First, let’s look at what accelerations is. Acceleration is proportional to force when the mass of an object is constant; and acceleration is the rate of change of velocity and has the same direction.
Point A: the ball is rolling down the slope. Its direction of motion is parallel to the slope and there is a gravitational force on the ball. The vector is parallel to the slope pointing down it.
Point B: this point is horizontal and there are no horizontal forces (if we ignore friction), so the acceleration would be zero horizontallly. However, if there is friction, it will tend to slow the ball down by creating a frictional force opposite to its motion to the right. Therefore there is a slight deceleration (negative acceleration) and this would be represented by a vector pointing backwards. Although the ball continues to move forward its velocity is decreasing so at some point at or near B the ball stops accelerating and starts to decelerate. We can say that the acceleration drops to zero before going negative for the next part of its journey.
Point C: the ball is affected by gravity pulling against its motion, so the overall acceleration is negative, slowing the ball down. Therefore the vector points backwards parallel to the slope.
Point D: the force of gravity is absent as the ball reaches the top of the slope so there is at most a frictional force opposing the motion and making the ball slow down a little. However, at some point at or near D, the deceleration will stop, so it becomes zero again as at B.
Point E: apart from friction slowing the ball down, there are no horizontal forces and the acceleration remains at zero, or small and negative (backwards) if friction occurs. If there’s no friction the ball proceeds at constant velocity.