<P> In physics, a quantum (plural: quanta) is the minimum amount of any physical entity involved in an interaction . The fundamental notion that a physical property may be "quantized" is referred to as "the hypothesis of quantization". This means that the magnitude of the physical property can take on only certain discrete values . </P> <P> For example, a photon is a single quantum of light (or of any other form of electromagnetic radiation), and can be referred to as a "light quantum". Similarly, the energy of an electron bound within an atom is also quantized, and thus can only exist in certain discrete values . The fact that electrons can only exist at discrete energy levels in an atom causes atoms to be stable, and hence matter in general is stable . </P>

The theory that light exists in quantized chunks was proposed in response to which of the following