Answer;
Measuring the radioactive decay of element isotopes.
The primary technique for determining the absolute age of a rock is the measuring the radioactive decay of element isotopes.
Step-by-step explanation;
The measuring of the radioactive decay of element isotopes to determine the age of rocks is called Radiometric Dating.
Radiometric dating is a method of determining the absolute age of an object such as rocks by comparing the relative percentages of a radioactive (parent) isotope and a stable (daughter) isotope.