The basic scientific principle behind a nuclear reactor is controlled nuclear fission (option 4). Nuclear fission refers to the splitting of the nucleus of an atom, typically a heavy element such as uranium or plutonium. In a nuclear reactor, this fission process is controlled in a way that allows it to release a large amount of energy in a controlled manner.

In a nuclear reactor, the fission process is initiated by bombarding the nucleus of a certain isotope with a neutron. This causes the nucleus to become unstable and split into two smaller nuclei, releasing additional neutrons and a large amount of energy in the process. These released neutrons can then go on to collide with other nuclei and continue the chain reaction.

The purpose of controlling this chain reaction is to ensure that the release of energy is gradual and controlled. This is achieved by using materials known as control rods, which can absorb the excess neutrons and adjust the rate of the reaction. By carefully managing the rate of fission, a nuclear reactor can generate the desired amount of energy while preventing the reaction from becoming uncontrolled and potentially leading to a dangerous explosion.

Therefore, the correct answer to the question is option 4: Controlled nuclear fission.