What remains unchanged in an organism after its death according to radiocarbon dating principles?

In the context of radiocarbon dating, the correct choice reflects that carbon-12 levels remain unchanged in an organism after its death. Carbon-12 is a stable isotope of carbon that does not decay over time. Even after an organism dies, the amount of carbon-12 present in its remains remains constant because it does not undergo radioactive decay.

In contrast, carbon-14, which is a radioactive isotope, begins to decay after the organism's death at a known rate. This decay allows scientists to estimate the time since death by measuring the remaining carbon-14 levels. Therefore, the carbon-14 levels change over time, leading to the conversion of this isotope into nitrogen-14.

Oxygen levels and nitrogen levels do not play a significant role in radiocarbon dating as they do not follow the same decay patterns as carbon isotopes. Hence, focusing on carbon isotopes, it is the carbon-12 levels that illustrate stability and remain constant following the death of the organism.