Under whose direction was the first nuclear reactor built?

The correct answer is Option C — Enrico Fermi. The first nuclear reactor, named Chicago Pile-1 (CP-1), was built under the direction of Italian-American physicist Enrico Fermi at the University of Chicago. On December 2, 1942, the first self-sustained controlled nuclear chain reaction was achieved here. Option A is incorrect: Albert Einstein is famous for E=mc² and the photoelectric effect, for which he won the Nobel Prize in 1921. While his letter to President Roosevelt in 1939 urged nuclear research, he did not directly lead or build any nuclear reactor. Robert Oppenheimer was the scientific director of the Manhattan Project and is known as the father of the atomic bomb, not the first nuclear reactor. Option D is incorrect: Ernest Rutherford discovered the nucleus of the atom and the proton. He died in 1937, before the first nuclear reactor was built in 1942.