The assertion is incorrect, but the reason provided is correct.

When the displacement of a body is directly proportional to the square of the time, it implies that the body is undergoing motion with constant acceleration, not uniform acceleration. Uniform acceleration refers to motion with a constant rate of change of velocity. In this case, the body's displacement would be directly proportional to time, not the square of time.

The reason provided, however, is correct. The slope of a velocity-time graph gives the acceleration of the body. In this scenario, if the displacement is proportional to the square of time, the velocity-time graph would be a straight line with a constant slope, indicating a constant acceleration.