Controlled nuclear chain reactions a. take place in nuclear reactors b. are always fusion reactions c. never produce radioactive by-products d. are characteristic of atomic bombs

a. take place in nuclear reactors

Controlled nuclear chain reactions are the reaction in nuclear reactors. The reaction is controlled to make its rate slower. This will make the reaction relatively safer, so the energy produced can be changed into electricity in power plant. When the reaction is not controlled, it could cause explosive like in nuclear bomb.

Hope the answer is b