If the displacement of an object is proportional to the square of time, then the object moves with
Uniform acceleration
Displacement:
Displacement is defined as the shortest distance between the initial point and the final point.
If the displacement of the object is directly proportional to the square of time, then
The velocity of the object is,
Thus, the acceleration of the object is,
Thus, the acceleration is constant.
Hence, option (b) is correct.