The correct option is C in all cases irrespective of whether the motion is one of uniform velocity, or of uniform acceleration or of variable acceleration
The area under the velocity-time graph between any two instant t=t1 and t=t2 gives the distance covered in time δt=t2−t1.
1. Straight line parallel to time-axis - uniform motion
2. Straight line inclined to the time-axis - non-uniform motion
Thus, area under v−t graph gives displacement for motion in uniform velocity, uniform acceleration, variable acceleration.