Einstein was awarded the Nobel Prize for:

Theory of photoelectric effect

The correct answer is option 2. Theory of photoelectric effect.

Albert Einstein was awarded the Nobel Prize in Physics in 1921 for his explanation of the photoelectric effect, which is a phenomenon where electrons are emitted from a material when it is exposed to light. This explanation was part of his groundbreaking work published in 1905, known as the "Annus Mirabilis papers," where he presented several revolutionary ideas in physics.

The photoelectric effect played a crucial role in the development of quantum theory. Einstein's explanation proposed that light is composed of discrete packets of energy called photons, and the energy of each photon is proportional to its frequency. This explanation helped establish the concept of wave-particle duality, which is central to quantum mechanics.

Einstein's work on the photoelectric effect laid the foundation for modern quantum theory and had significant implications for the understanding of the behavior of light and matter at the atomic and subatomic levels. It also contributed to the development of technologies such as solar cells and photodetectors, which rely on the principles of the photoelectric effect.

While Einstein is best known for his theory of relativity, his work on the photoelectric effect was considered groundbreaking and earned him the Nobel Prize, highlighting the profound impact of his contributions to physics.