Einstein was awarded the Nobel Prize for which one of the following theories?

The correct answer is B) Theory of photo-electricity. Einstein's 1905 light-quantum paper explained the photoelectric effect, and the Nobel Committee cited his 'discovery of the law of the photoelectric effect' when awarding the 1921 prize (presented in 1922). Light arrives in discrete packets (photons), and the energy of an ejected electron depends on the frequency of light, not its intensity. That is the core idea.

Why the others are wrong:

• A) Theory of relativity: Special (1905) and general (1915) relativity made him famous and are central to modern physics, but the Committee deliberately did not reward them, as they were not yet considered fully verified by experiment at that time.
• C) Theory of specific heats: Einstein did work on the specific heat of solids (1907) using quantum ideas, but it was not the basis of the prize citation.
• D) Theory of Brownian motion: His 1905 explanation of Brownian motion supported the existence of atoms and molecules, but it was not the prize citation either.

Core concept: quantisation of light and the photoelectric effect, which underpins quantum mechanics and later photovoltaic technology, a useful Mains link for science and technology questions.

Trap: Students pick relativity because it is the most famous work. Remember that the Nobel citation specifically named the photoelectric law.