The correct answer is option 1. • Carbon Dating • Carbon-14 is a weakly radioactive isotope of Carbon; also known as radiocarbon, it is an isotopic chronometer. • C-14 dating is only applicable to organic and some inorganic materials (not applicable to metals). • Gas proportional counting, liquid scintillation counting and accelerator mass spectrometry are the three principal radiocarbon dating methods. • What is Radiocarbon Dating? • Radiocarbon dating is a method that provides objective age estimates for carbon-based materials that originated from living organisms. • An age could be estimated by measuring the amount of carbon-14 present in the sample and comparing this against an internationally used reference standard. Hence, statement b is correct. Explanation & Key Takeaways: • Basic Principles of Carbon Dating • Radiocarbon (carbon 14) is an isotope of the element carbon that is unstable and weakly radioactive. The stable isotopes are carbon 12 and carbon 13. • Carbon 14 is unstable and carbon 12 is stable, thus, with an increase in age the ratio of 14C to 12C changes continuously for dead bodies or plants. Hence, statement a is correct. • Carbon 14 is continually being formed in the upper atmosphere by the effect of cosmic ray neutrons on nitrogen 14 atoms. • It is rapidly oxidized in air to form carbon dioxide and enters the global carbon cycle. • Plants and animals assimilate carbon 14 from carbon dioxide throughout their lifetimes. • When they die, they stop exchanging carbon with the biosphere and their carbon 14 content then starts to decrease at a rate determined by the law of radioactive decay. • Radiocarbon dating is essentially a method designed to measure residual radioactivity.