Answer: Carbon 12
Explanation: Carbon 12 is the most naturally occurring stance isotope of carbon, constituting over 98% of available carbon. The importance of carbon cannot be overestimated in conjunction with it's contribution to the green house effect. Excessive release of carbon store in rocks, exhaust from burning coal, fuel and other petrochemicals contribute to the release of carbon and excessive depletion of the ozone layer. In a bid to maintain carbon composition, increase in carbon burial on continental or oceanic crust is accompanied by the depletion in carbon 12 of both atmospheric and oceanic carbon dioxide.