Explanation

(d) Since, no external force is present on the system so, conservation principle of momentum is applicable
$\therefore{\overrightarrow{p}}_{i} = {\overrightarrow{p}}_{f} = {\overrightarrow{p}}_{1} + {\overrightarrow{p}}_{2}$
$\therefore\ {\overrightarrow{p}}_{1} = - {\overrightarrow{p}}_{2}\ \ \ \ \ \ \ \ ({\because\overrightarrow{p}}_{i} = 0)$
$\therefore\ \ \left| {\overrightarrow{p}}_{1} \right| = \left| - {\overrightarrow{p}}_{2} \right|$
$\therefore\ \ {\overrightarrow{p}}_{1} = {\overrightarrow{p}}_{2}$
From this point of view, it is clear that momenta of both particles are equal in magnitude but opposite in direction
Also, friction is absent. So total mechanical energy of system remains conserved