Which concept states that mass and energy are interchangeable forms of the same concept?

Mass-energy equivalence is a fundamental principle in physics formulated by Albert Einstein, encapsulated in the famous equation (E = mc^2). This equation highlights that mass can be converted into energy, and conversely, energy can manifest as mass. This concept reveals that mass and energy are not distinct entities but two forms of the same fundamental quantity.

The principle of mass-energy equivalence explains phenomena such as nuclear reactions, where small amounts of mass are converted into significant amounts of energy. This underpins the operation of atomic bombs and nuclear reactors, where the released energy is derived from mass loss.

The other concepts mentioned, such as the law of conservation, primarily refer to the idea that certain quantities remain constant in isolated systems. Energy conservation focuses specifically on the total energy in a closed system remaining constant, while the kinetic energy principle relates to the energy an object possesses due to its motion. These other concepts do not address the interchangeability of mass and energy in the way that mass-energy equivalence does. Therefore, it is this principle that underlines the relationship between mass and energy as interchangeable forms, confirming the correct answer.