A nuclear reactor is said to have become ‘critical’ when A. It stops due to malfunctioning B. It starts emitting dangerous radioactive radiations C. It is shut down to avoid explosion D. It is ready to produce controlled energy

The correct answer is: D. It is ready to produce controlled energy.

A nuclear reactor is said to have become “critical” when the rate of nuclear fission in the reactor is self-sustaining. This means that the number of neutrons produced by fission is equal to the number of neutrons absorbed by the fuel. When a reactor is critical, it is producing a steady stream of heat, which can be used to generate electricity.

Option A is incorrect because a nuclear reactor does not stop due to malfunctioning when it becomes critical. In fact, the opposite is true: a nuclear reactor becomes critical when it is operating correctly.

Option B is incorrect because a nuclear reactor does not start emitting dangerous radioactive radiations when it becomes critical. In fact, the amount of radiation produced by a nuclear reactor is actually quite low when it is operating correctly.

Option C is incorrect because a nuclear reactor is not shut down to avoid explosion when it becomes critical. In fact, a nuclear reactor is only shut down if there is a problem with the reactor, such as a loss of coolant.

I hope this helps! Let me know if you have any other questions.