Answer is: A. the rate of decay for certain isotopes.
For example, radiocarbon dating is called Carbon-14 dating.
Carbon-14 is a radioactive isotope of carbon, with a half-life of 5730 years.
Carbon bound in fossil fuels has different isotopes than carbon in atmosphere. Fossil fuels are underground and cosmic rays do not affect carbon in them same as atmosphere carbon. Carbon in fossil fuels that is underground longer than ten thousand years has only C-12 isotope and not C-14.