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

A graph plotting distance against time is called the distance-time graph. Here, distance is plotted on the y-axis and time along the x-axis. For an object moving at a constant speed, its distance-time graph will be a straight line.

The slope of a distance-time graph gives the speed of a body. Also by using a distance-time graph we can find the distance moved by an object at any instant of time.

Here,

The distance-time graph of options A and D represents straight lines, which indicates that the truck is moving at a constant speed.

The distance-time graph of option B has a straight line parallel to the time axis, which indicates that the truck is not moving. 

i.e., the truck is at rest.

The distance-time graph of option C has a curved line, which indicates that the distance is not changing equally in equal intervals of time. This means the truck is moving at a speed that is not constant.

So option C is the answer.