What Is the Meaning of the Term "Critical Mass

What is the meaning of the term "critical mass"?

A) This refers to the mass of the "critical" elements in a reactor, i.e., the uranium or plutonium.
B) This refers to the minimum amount of fissionable material required to sustain a chain reaction.
C) This refers to the maximum amount of fissionable material required to sustain a chain reaction.
D) This is the amount of mass needed to make a power reactor economically feasible.
E) This is the material which is just on the verge of becoming radioactive.