Why Correct: Albert Einstein received the 1921 Nobel Prize in Physics specifically for his discovery of the law of the photoelectric effect, which he explained using quantum theory in 1905.
Distractor Analysis: Special theory of relativity, published in 1905, revolutionized physics but was not cited for the Nobel. General theory of relativity, completed in 1915, was too controversial at the time for Nobel recognition. Brownian motion explanation, also from 1905, provided evidence for atomic theory but was not the prize-winning work.
Takeaway: The Nobel Committee explicitly mentioned "for his services to Theoretical Physics, and especially for his discovery of the law of the photoelectric effect" in the award citation.