Final answer:
In modern relativity, the classical laws of conservation of energy and mass are combined into the law of mass-energy conservation. Einstein's theory introduced mass-energy equivalence, modifying the conservation principles to accommodate the fact that mass can be converted into energy and vice versa.
Step-by-step explanation:
The classical laws of conservation of energy and mass have been modified by modern relativity theory. In classical physics, it was thought that mass and energy were conserved separately. However, Einstein's theory of relativity introduced the concept of mass-energy equivalence, encapsulated by the famous equation E=mc^2. This equation tells us that mass can be converted into energy and vice versa, meaning that the total mass of a closed system may not remain constant if some of the mass is converted to energy (or the other way around).
Therefore, in modern relativity, it is not accurate to say that mass is conserved; instead, we consider that mass and energy together are conserved. This is because the total energy of a system, which now includes any energy equivalent of mass, remains constant in all processes. The law of conservation of energy has also to be modified slightly to acknowledge that energy can manifest as mass, and as such, the total energy of an isolated system remains constant.
So, while classically energy and mass were considered separately conserved, modern relativity combines the two into a single law of conservation. Therefore, the correct answer to the original question is (c) both energy and mass are conserved, with the understanding that they are conserved as a single entity: mass-energy.