Final answer:
The correct answer is A) Critical Mass, which is the minimum amount of fissionable material necessary for a self-sustaining nuclear fission chain reaction.
Step-by-step explanation:
The amount of an element that must be present at a given density in order for a nuclear reaction to be self-sustaining is the critical mass. This is the minimum amount of fissionable material needed to maintain a self-sustaining nuclear fission chain reaction. When the critical mass is achieved, the number of neutrons produced by fission is enough to sustain the reaction, as they are captured by other fissionable nuclei, splitting them and releasing more neutrons. If the mass is too low (subcritical mass), the chain reaction will not be self-sustained; if the mass is too high (supercritical mass), the rate of fission will increase exponentially.
In terms of options provided: A) Critical Mass, B) Atomic Mass, C) Isotopic Mass, D) Molecular Mass, the correct answer is A) Critical Mass.