What Is the Meaning of the Term "Critical Mass" in Regard

What is the meaning of the term "critical mass" in regard to nuclear fission?

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