What Does the Principle of Conservation of Energy Indicate for the Net

What does the principle of conservation of energy indicate for the net work done on a system?

A) Energy is never lost.
B) Energy cannot pass from one system to another during a collision.
C) The net work done on a system is converted into kinetic and potential energy such that the total energy of the system remains constant throughout the motion.
D) The net work done on a system is converted into kinetic and potential energy such that the total energy of the system remains constant throughout the motion, only when there are no external forces acting on the system.