Einstein was awarded the Nobel Prize for which of his theories?

1. A

   Theory of relativity

   The theory of relativity made Einstein famous but did not win his Nobel Prize. Relativity is the prestige distractor, not the keyed award.

2. B

   Theory of photoelectricity

   (b) Official key: Einstein’s Nobel Prize (1921, awarded 1922) was for the law of the photoelectric effect. Among relativity, photoelectricity, specific heats and Brownian motion, photoelectricity is the official prize theory.

3. C

   Theory of specific heats

   Work on specific heats is part of Einstein’s early quantum papers, not the Nobel citation. Specific heat is not the key.

4. D

   Theory of Brownian motion

   Einstein explained Brownian motion (1905), confirming atoms; that paper did not carry the Nobel. Perrin’s experiments completed that story. Brownian motion is not the keyed prize.

Summary. Official key is (b). Einstein’s Nobel is for the photoelectric effect, not relativity. Specific heats and Brownian motion are other 1905-era contributions. Keep photoelectricity. The official letter is (b).