The correct answer is 3) the amount of parent and daughter material trapped in the rock.
Potassium–argon dating (K–Ar dating), is a method which is used in archaeology and it is based on measurement of the product of the radioactive decay of an isotope of potassium (K) into argon (Ar). The decay product Ar is able to escape the liquid rock. But, when the rock solidifies (recrystallizes) Ar starts to accumulate. To calculate time since recrystallization you must measure
the ratio of the amount of Ar accumulated to the amount of K that remained.