A. the external force is 0. The resultant force of 0 does not mean that the external force does not do work, because the external force torque is not necessarily 0, for example, the external force accelerates a wheel rotating on a fixed axis and the external force does work. At the same time, non-conservative internal forces may do work and convert mechanical energy into other energy.

B. if the external forces work together, no work will be done. Non-conservative internal forces may do work and convert mechanical energy into other energy.

C. neither external force nor non-conservative internal force works. Conservative internal force will only convert energy between kinetic energy and potential energy, and will not affect the mechanical energy of the system. At this time, the mechanical energy of the system is conserved.

D. neither external force nor conservative internal force works. Non-conservative internal forces may do work and convert mechanical energy into other energy.

So choose C.