For what significant theory is Albert Einstein best known?

Albert Einstein was a German-born theoretical physicist who developed the theory of relativity, one of the two pillars of modern physics. His work is also known for its influence on the philosophy of science. He received the 1921 Nobel Prize in Physics for his discovery of the photoelectric effect.

For what significant theory is Albert Einstein best known?

1. A.Quantum Mechanics
2. B.Theory of Gravity
3. C.Theory of Relativity
4. D.Big Bang Theory