Here's why:

* Radiometric dating uses the decay of radioactive isotopes within the rock. These isotopes decay at a known and constant rate, acting like a natural clock. By measuring the ratio of parent isotope to daughter isotope, scientists can calculate the time elapsed since the rock crystallized.

Here's why other options are less accurate:

* Index fossils: While useful for determining relative age (older/younger), index fossils don't provide an absolute age in years.

* Stratigraphy: Examining rock layers helps establish relative age but doesn't give a precise numerical age.

* Paleomagnetism: Records the Earth's magnetic field at the time of rock formation, but doesn't directly determine absolute age.

In summary, radiometric dating is the most reliable method for determining the absolute age of an igneous rock.