Question

Definition: this law states that, in any process, energy is neither created nor destroyed. it can only be converted from one form to another.

Answer 1

answer: the law of conservation of energy.

This law applies to isolated system and means that the total energy in a given frame of reference is conserved over time. Its not created or destroyed and can only be converted from one form to another.
The conservation of energy can be proven by the Noether theorem.