What is the result of an uncontrolled chain reaction
a small explosion
a small implosion
a large explosion
a large implosion

A chain reaction is a nuclear reaction that occurs when sufficient unstable nuclei are present so that when one splits (decays), its products hit other unstable nuclei, causing them to decay and initiating a chain reaction.

If this reaction goes uncontrolled, a massive explosion occurs. Such a reaction is the type that went uncontrolled when nuclear bombs were dropped over Hiroshima and Nagasaki in World War II. A chain reaction is much like dominoes all in a row; when one goes down, they all go down in order. The only way to block the chain reaction is to pull out a domino or to block the fall so the "chain" is broken.

The amount of fissionable material needed to accomplish a spontaneous chain reaction is called the critical mass. The critical mass for U-235 is about 1 kg. This explosive fission process takes place quickly, and an incredible amount of energy is released when it occurs.

So it would be C.

the answer for sodium is 0.186

the answer for rubidium is 0.251