Where is the energy produced by a controlled chain reaction used?

The correct answer is Option B — Nuclear Reactor. This energy is harnessed in nuclear reactors to produce electricity. The chain reaction is kept controlled (k=1) using control rods made of cadmium or boron. Option A is incorrect: Nuclear weapons use uncontrolled chain reactions, not controlled ones. The energy in weapons is released explosively. Option C is incorrect: Chemical factories use chemical reactions, not nuclear chain reactions. The energy scale of nuclear reactions is millions of times greater than chemical reactions. Option D is incorrect: Solar panels convert sunlight (photovoltaic effect) into electricity. They have nothing to do with nuclear chain reactions. Nuclear reactors work on the principle of controlled fission where the energy heats water to produce steam, which drives turbines to generate electricity.