Which common and stable isotope is the standard against which the atomic weights of all other elements are measured?

Question

In Radiocarbon dating method to estimate the age of organic matters recovered from the archaeological sites, the residual quantity of which carbon isotope in the matter is measured to determine its age ?

Answer 1

Carbon-12
Carbon-12 contains 6 protons, 6 neutrons, and 6 electrons, and constitutes 98.93% of the world's elemental carbon. The decayed presence of carbon-14 in organic materials is the basis of the radiocarbon dating method pioneered in 1949. Until 1959/60 oxygen-16 was the substance used by physicists to define a mole.