Answer:
C. the age of rock or fossil
Step-by-step explanation:
Half life of a radioactive isotope tells us that how much time the isotope will take to decay to 50% of its original mass. Using the half life, the decay constant of the isotope can be calculated. Employing the decay rate, with the method known as radiometric dating, scientists calculate the age of rocks and fossil fuels. Age of earth is also approximated using the same technique.
Therefore, the option C is the correct answer.