Final answer:
Relativistic momentum is conserved only in closed systems with no external forces. In cases where external forces exist, conservation of momentum can only be applied by expanding the system to include the source of the force.
Step-by-step explanation:
The conservation of momentum is a fundamental principle in physics that applies to both classical and relativistic systems. According to this principle, the total momentum of a closed system (a system with no external forces) is conserved. Therefore, the correct answer to whether relativistic momentum can be conserved when an external force acts on a system is b) No, conservation of relativistic momentum requires no external forces.
In the context of relativity, momentum is conserved within a closed system, meaning no external forces are acting upon the objects within that system. If an external force is applied, it will change the total momentum of the system unless the system's definition is expanded to include the source of the external force. In such an expanded system, the concept of conservation of momentum can still apply. Perpendicular or parallel forces do not change this condition, as the presence of any external force will affect the system's momentum.