The difference between a nuclear reactor and an atomic bomb is that
A. no chain reaction takes place in nuclear reactor while in the atomic bomb there is a chain reaction
B. the chain reaction in nuclear reactor is controlled
C. the chain reaction in nuclear reactor is not controlled
D. no-chain reaction takes place in atomic bomb while it takes place in nuclear reactor

Hint: In the solution we will be using the term nuclear fission which is important in the study of nuclear reactor and atomic bomb.

Complete step by step solution:
In nuclear fission, the breaking of nuclear fuel into smaller particles and thus liberating a higher amount of heat.

Nuclear reactor is a part of a nuclear power plant whose primary function is to carry out nuclear fission of the uranium fuel of different forms in a controlled manner. The rate of reaction is controlled by moderator, hence it can be said that the reaction is a controlled chain reaction. The high amount of heat energy released due to fission of nuclear fuel is transferred to the fluid, mainly water which is utilized in the subsequent steps in a nuclear power plant to produce electricity.

In atomic bombs the energy released is due to nuclear fission or both nuclear fusion and fission which is utilised to destroy or nullify the target. In an atomic bomb, there is no moderator which can moderate or control the chain reaction, hence reaction takes place in an uncontrolled manner therefore liberating a high amount of heat.

From the above explanation, we could clearly understand that in both nuclear reactor and atomic bomb reaction is a chain reaction but with a difference that in nuclear reactor it is of controlled type whereas in atomic bomb it is uncontrolled.

Therefore, the option B. That is, the chain reaction in a nuclear reactor is controlled is correct.

Note: Have a clear understanding and difference between nuclear fission and fusion.