The photoelectric effect provides the most direct evidence of the photon model of electromagnetic radiation. Hence the correct option is c.
The photoelectric effect serves as a fundamental experiment that directly supports the photon model of electromagnetic radiation. In this phenomenon, light incident on a metal surface induces the ejection of electrons from the metal. The key insight from the photoelectric effect is that electrons are emitted almost immediately, with no delay, once the light reaches a certain threshold frequency.
This behavior is best explained by the photon model, proposed by Albert Einstein, where light is considered as discrete packets of energy called photons. The energy of each photon is directly proportional to its frequency. In the photoelectric effect, electrons absorb energy from individual photons, and only when the energy of a photon exceeds the work function of the metal does an electron get ejected.
The observations in the photoelectric effect align with the quantized nature of photons, providing clear and direct evidence for the existence of these discrete energy packets. This experiment played a crucial role in the development of quantum theory and the acceptance of the particle-like nature of light. Other experiments, such as Rutherford scattering, constant pressure calorimetry, and two-slit diffraction, do not directly address the quantized nature of electromagnetic radiation as explicitly as the photoelectric effect does. Hence the correct option is c.