Which of the following distance-time graphs shows a truck moving with speed which is not constant?

(i)

(ii)

(iii)

(iv)

In option (iii), the distance time graph is not a straight line, instead it is a curve. The distance time graph for an object moving with a constant speed is straight line. This means that the speed is not constant is changing. So, the distance-time graph of option (iii) shows a truck moving with speed which is not constant.