The value of the momentum for a system is the same at a later time as at an earlier time if there are no a) collisions between particles within the system. b) inelastic collisions between particles within the system. c) changes of momentum of individual particles within the system. d) internal forces acting between particles within the system. e) external forces acting on particles of the system.

The conservation of momentum states that the total momentum of a closed system remains constant if no external forces act on the system. In other words, the sum of the momenta of individual particles within the system remains the same over time if no external forces influence the particles.

Now, let's evaluate each given option: a) Collisions within the system might transfer momentum between particles, but the total momentum will remain constant, so this option is not right. b) Inelastic collisions between particles might cause energy dissipation as heat, but they still conserve the overall momentum of the system. Therefore, this option is incorrect as well. c) If there are no changes in the momentum of individual particles, the total momentum will definitely be conserved. This option seems correct, but we should evaluate other options as well. d) Internal forces acting between particles only redistribute the total momentum among the particles, they do not change the overall momentum of the system. Thus, this option is not correct. e) External forces acting on particles of the system can change the total momentum, but the question is about scenarios when the momentum remains constant. Hence, this option is not correct.

Based on the evaluation of each option and principles of conservation of momentum, the correct answer is (c) changes of momentum of individual particles within the system.