A - Photoelectric Effect
B - Relativity
C - Gravitational Pull
D - Supersymmetry
Answer: A - Photoelectric Effect
Though Einstein is most famous for his theory of relativity, the public still wasn’t totally sold on the theory in 1921. That's why Einstein won his Nobel Prize for explaining the photoelectric effect — though he went ahead and lectured on relativity in his acceptance speech anyway.
If metal electrodes are exposed to light, electrical sparks between them occur more readily. For this "photoelectric effect" to occur, the light waves must be above a certain frequency, however. According to physics theory, the light's intensity should be critical. In one of several epoch-making studies beginning in 1905, Albert Einstein explained that light consists of quanta - "packets" with fixed energies corresponding to certain frequencies. One such light quantum, a photon, must have a certain minimum frequency before it can liberate an electron.