The x−t graph shown, is parallel to time axis. This means that the object is at rest. Hence, the velocity of
the object is zero for all time instants. Hence, v−t graph coincides with the time axis as shown in graph (a).
In graph (b) velocity is increasing uniformly with respect to time.
In graph (c), straight line represents constant acceleration
while in graph (d) acceleration is first increasing then decreasing.