Question

Controlled nuclear chain reactions ____.

Answer 1

Controlled nuclear chain reactions: 
a. take place in nuclear reactors
When one or more subsequent nuclear reactions came from a single nuclear reaction makes that leads to the probability of a self-propagating series of these reactions then a nuclear chain reaction occurs. 

Answer 2

Answer: 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.