The definition of a limiting reactant can be summarized as: athe reactant that is used up
bthe greater amount produced
cthe reactant that is not fully used
dthe smaller amount produced

Explanation:

A limiting reactant in a chemical reaction is one which puts limits on amount of product  produced out of the reaction. The process of chemical reaction is stopped when the limiting reactant is fully used up.

Example:

Suppose you have

5 gram of oxygen and 10 gram of hydrogen to make any chemical product say y

since

amount of oxygen is only 5 gram thus, when all of 5 gram of oxygen will be used up then no further reaction will take place even though amount of hydrogen is present to react.

Thus, we can say that

oxygen is limiting the production of product y.

oxygen is fully used until the reaction stops.

Thus, in this case oxygen is limiting reactant.

Based on above discussion

the correct option for this problem is

a the reactant that is used up

14 days is the half-life of phosphorus-32