The graph in the question that represents an object with constant acceleration is : Graph A
Acceleration is the change in velocity with time. For graph A the acceleration remained constant as the velocity of the object did not change with the change in time. As the time changes from 4 to 9 seconds the velocity of the object remained at 40 before decelerating to 0. This shows that the acceleration of the object was constant.
Hence we can conclude that The graph that shows an object with constant acceleration is : Graph A
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