A girl walks along a straight line to drop a letter in the letterbox and comes back to her initial position. The displacement versus time graph is shown

 Which of the following is its velocity-time graph?

In the following, the velocity-time graph is not determined.
From the given graph the velocity-time graph can be drawn as 

Here the girl travels in a straight line and returns back to her initial position. So the journey upwards will have a positive velocity and the journey backwards will have a negative velocity as the direction of motion is reversed. The girl has zero velocity at the midpoint of the displacement time graph. The velocity for the first half of the journey will be
$\mathrm{v}=\frac{\mathrm{displacement}}{\mathrm{time}}$
$=\frac{100}{50}$
$=2 \mathrm{m}/\mathrm{s}$
And that of the return journey will be $\mathrm{– }2 \mathrm{m}/\mathrm{s}.$