(d) Linear momentum is not conserved for any of the situations described in (a), (b) or (c).
Work Step by Step
Linear momentum is conserved for all forces applied, as long as the net force applied is zero. An unbalanced force accelerates a body, which changes velocity. Linear momentum thus, changes and is not conserved. For conservative forces acting on body, the mechanical energy is conserved but the velocity may change, leading to a change in momentum. For example, a ball thrown up in the sky.