a) The spring will continue to compress until the two blocks acquire common velocity. So some of kinetic energy of block M, store into P.E. and some part of it stores into K.E. of block M2. So option (a) is incorrect. b) As surface are frictionalless momentum of the system will be conserved. So option (b) is also incorrect c) The two bodies of equal mass exchange their velocities in a head on elastic collision between them. So, if spring is massless, the final state of the M, is state of rest. d) Since there is a loss of K.E when the blocks collides on the rough surface. Hence, the collision is inelastic.