The difference between a nuclear reactor and an atomic bomb primarily lies in the control of the chain reaction. In option 1 and 4, it`s stated that no chain reaction takes place in either the nuclear reactor or the atomic bomb, which is incorrect. Both devices undergo nuclear chain reactions. Option 3 is wrong because the chain reaction is indeed controlled in a nuclear reactor, preventing it from exploding like an atomic bomb. The correct answer is option 2 because nuclear reactors are designed to control the chain reaction very precisely, allowing the slow release of energy over a long time, which is then converted into electricity. On the other hand, the chain reaction in an atomic bomb is uncontrolled and allows for a sudden large release of energy, resulting in an explosion.