Scientists estimate the age of a rock or fossil using the known decay rate of certain isotopes, a method called

Correct answer to question 187 (General Science): radiometric dating

Method

Radiometric dating measures how much of a radioactive isotope in a sample has decayed into a stable form; because the decay rate is constant, that ratio reveals the sample's age. Carbon-14 dating is one specific version, useful only for material under about 50,000 years old.