Final answer:
Carbon-14 (C14) is used to determine the age of an organism through carbon dating.
Step-by-step explanation:
Carbon-14 (C14) is used to determine the age of an organism through a process called carbon dating. When an organism is alive, it constantly replenishes its C14 supply through respiration, resulting in a constant amount of the isotope. However, when the organism dies, it no longer takes in carbon from the environment and the C14 begins to decay. By examining the amount of remaining C14 against its known half-life of 5,730 years, scientists can approximate the date at which the organism lived on Earth.