Explanation

(b) From the graph, it is a straight line so, uniform motion. Because of impulse direction of velocity changes as can be seen from the slope of the graph.
$Initial\ velocity\ \mathbf{v}_{1} = \frac{2}{2} = 1\ {ms}^{- 1}$
$Final\ velocity\ \mathbf{v}_{2} = \frac{- 2}{2} = - 1\ {ms}^{- 1}$
$\mathbf{p}_{i} = m\mathbf{v}_{1} = 0.4\ Ns$
$\mathbf{p}_{f} = m\mathbf{v}_{2} = - 0.4\ Ns$
$\mathbf{J =}\mathbf{p}_{f} - \mathbf{p}_{i} = - 0.4 - 0.4$
$= - 0.8\ Ns\ \ (\mathbf{J =}impulse)$
$\mathbf{\therefore\ \ }\left| \mathbf{J} \right|\mathbf{=}0.8\ Ns$