The answer is:
Atmosphere
-CARBON DIOXIDE FROM THE ATMOSPHERE.
The Explanation:
The source of the carbon-14 on the earth is the effect of cosmic rays with nitrogen on the atmosphere layer, when the cosmic rays interact with the atoms of oxygen and nitrogen will produced a lighter and smaller particles like X rays and alpha particles and gamma rays, and by the ionization of the oxygen and nitrogen at the atmospheric layer and by a number of chemical reaction will lead to carbon-14 according to this reaction equation:
n + 14N → p + 14C