The hill on which the ball roll down with increasing speed and decreasing acceleration is the middle hill in the diagram.
Your question is not complete, it seems to be missing the diagram I added.
Velocity is the change in displacement per change in time.
Acceleration is the change in velocity per change in time of motion.
In the given diagram;
Thus, the hill on which the ball roll down with increasing speed and decreasing acceleration is the middle hill in the diagram.
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