The controlled release of nuclear energy in a reactor is accomplished by: allowing the critical mass of fuel to react controlling a chain reaction controlling fission without a chain reaction all of the above

All three of those statements are true ways to say the same thing.

The reactor holds a critical mass of fuel, in a space small enough

for a chain reaction to begin, but places obstacles in the path of

emitted neutrons, so that enough of them get through to keep the

chain reaction going but not enough of them get through to allow

the chain reaction to accelerate and the whole thing to go blooey.